two questions:
which one is the best overall?
which one is the best for a beginner?

Best for beginner? Probably SE4, though both games take time to learn. I was lucky, when MOO3 came out that was my first 4x and I smashed that difficulty curve so hard as a 12yr old it hurt my brain. I eventually got to where I could fight aliens and win a few times, then found SE4. So I found it easy from the get-go, but I am unsure if you are familiar with 4x's or not. If this is your first 4x I would suggest SE4 over SE5. Both are not bad choice.

What is the best? Space Empires 5 I consider the best due to proper 3d combat, hex instead of square maps, and many small refinements you won't notice unless you played SE4 for a bit. The AI is fairly dumb though, with resource bonuses and the hard setting with the AI all set to team up against you as a lone player I being to find challenge occasionally but end up wiping them out one at a time. The only real challenge comes from their massive block of combined intelligence services attacking me and having to make an excessive amount of intel purely to counter all of them at once.

Space Empires 4 the AI can threaten you as long as it's not one of the minor races locked to a single system. Set them to hard with a small resource bonus and you will want to rush mines and decent weapons technology to keep fighting them off. A few times with the AI set to all team up against the human player I got pushed into an honest losing situation. SE5 I would lose a few systems at most maybe but was established enough to have several more, and fight back and win. Mind you SE5 came after SE4 and I was more experienced by the time SE5 came out.

SE5 has a better tech tree in that you can keep researching stuff for a lot more gradient levels. Most weapon techs though after they reach max range only get like +5dmg per level or something minor along those lines yet each type of weapon has it's use even in later stages of the game.

DUC - 30kt space used, 80 range, uses small amount of ordnance each shot, reliable

Meson - 20kt, 70 range, less damage but uses supplies only no ordnance (meaning solar cells can let a ship keep fighting after running out, ordnance can only be resupplied from supply depots or taken from other ships)

Anti-Proton beam - 30kt, 90 range, very little damage at 90, still less damage than mesons and DUC at half range, only ramps up really close so it can tag stuff but not hurt it much, only uses supplies

This is a small generalization and based only on the weapons techs and assuming they are equal level. This doesn't count how the meson-based point defense has double the rate of fire with nearly the same damage output as standard point defense, or how the small DUC for ground units is the most effective and highly damaging weapon and only 3kt to boot compared to others that do less damage and are bigger. Point defense, fighter, and ground unit variants of these weapons come into play so it's worth looking into each weapon tech at some point in your game, but to start off I'd suggest sticking the DUC's unless you want to focus on a fleet that can be resupplied purely off solar panels for long-distance fighting with no friendly systems nearby.

There is a lot more you can choose with SE5 techs, like how focusing on robotics can make your mining/farming worlds more productive at higher levels or computers can allow you to massively boost intel and research productivity. Usually each of these techs is expensive enough you can choose one or a few in the early game, and it's usually wise to level up a few primitive ones like DUC's and sensors first so you can at least explore and defend yourself. As you get larger you can boost your civilization's effectiveness with the same colonies by having better miners, research and intel facilities that can have double the income or more if you focus on that enough. But there is a catch, don't lag behind on weapons and defense tech, or ship frames and stuff to explore with. Best research and intel in the game does nothing if the enemy can touch you but you cannot touch them with your ships. SE5 has more variance in what path you can choose to go down in your tech tree.

Do I still boot up SE4? Yes, it's tighter and has it's own balance. It's own game direction. It still gives you lots of freedom to choose how to go about winning compared to many other games. There is no ordnance, everything is generally supplies, good and bad. I like it for being simplified but also don't like it because I love that nuance of potentially having a fleet that can re-supply in space via solar panels, there is tactical viability for long-term fights in that manner. SE4 has tighter AI. I never played mods for either game, when I was a teen of course the AI were challenging enough and now as an adult I play with the AI getting resource bonuses on hard with the AI-vs-Human setting to force all of them to be allied against you. Since this released on GOG I might look at Kwok's balance mod and what he does with the AI, personally I wish he would just release his upgrades to the AI without the "balance" part. I want the base game with just a better AI.

SE5 you will likely have to change video input so it plays windowed in a modern OS, otherwise you get massive mouse lag. I still play it with a movie running on the other part of my screen. SE4 full-screen still looks fine, and SE4 can run on a very old potato like a 10yr old Intel-based laptop no problem.

I still play both, I still like both, both are variations of a similar formula. I find them both to be better than other 4x games, stuff like Star Ruler or GalCiv do some things right and other things wrong. Or make certain things too simplified or esoteric. SE series you have the complexity that you can capture alien populations and put them on worlds with their atmosphere to double how many buildings your planets can have, sort of collecting aliens like pokemon to make your colonies more effective. You can plant minefields on a warp point to hit them as they come in or surrounding a warp point so they can jump in, you can see them, then you can cripple them on the way to their nearest target. You can build weapons platforms or make a bunch of fighters on each planet and launch them to intercept fleets before they get near said planet. Or you can keep it simple, build big ships and smash enemy. You can even go complex on ships, like adding small boarding parties to every ship and choosing to capture the last enemy ship in each engagement by having your entire fleet half fire and dump their boarding parties on the target (since a single 20kt boarding party unit is not enough to overtake even a frigate, you need multiples on same ship or lots of various ships). Most people just put a few guns and armor on frigates, but you could put a fighter bay and 3 or so fighters in some frigates instead. Making smaller cheap carriers before you even unlock the larger proper carrier frames can make sense since frigates can go full-speed and get a defense bonus for being smaller. You can choose to just keep ships simple like many new players and just load up on guns and missiles, or dedicate gunboats and missile boats.

This is what is great about Space Empires 4/5, if played simply they are still great but you can keep advancing the game into more complex concepts as you keep playing it. The only other 4x game I play as much is Alpha Centauri, and that is odd since I cannot stand the Civ series. I don't like GalCiv much at all but still play GC2 quite a bit, but then for some reason I cannot stand Stellaris and Endless Space 1/2 at all. Not at all. Not sure why, I just can't stand them. I can tell they are made with quality, it just doesn't....feel right. Focuses on wrong things, forced limits on how big you can make your empire, it seems they force you to play a certain way. Space Empires 4/5 do have a combat focus but that is how it is, even at peace neighbors test borders at times. Your job is to keep your empire competitive, and at some point that means warfare. Space Empires doesn't shy from that, you can go full-warmonger without crazy downsides tacked on for no reason like other games do, or you can reap the benefits of sparing an alien population by letting them populate your methane/hydrogen planets for example. There is benefit to not going full warmonger and killing all aliens. These games give you carrot or more carrot for good decisions, not stick if you don't play the exact way the devs want you to. That's why I find Space Empires to be the best to this day.