what skills should I pick for my 4 characters?

The general answer: A little of everything.
Maybe the only special tip worth to say (and not say in the game) is that eventually you will need the 3 skills related to conversation for unlock options with a single option.

Don't really agree with a little of everything. The conversation skills are useful, especially Smart Arse and Kiss Arse. It's better to specialise some characters in a few skills, then let most be decent at everything. Some skills don't bring all that much, but for example Trap Disarming is very useful as there are traps on just about everything it seems (it's called Demolitions for some reason).

Other skills it's useful to have high skills in somewhere in your party is
Field Medic
Surgeon
Perception (very important)
Leadership (also important I'd say, as it boosts attack and such in your party)
Weaponsmithing
Toaster Repair
Computer Science

You has 4 PC plus +3 NPC for making specialists in different skills. That is why I'm saying " a little of everything". For me, there is not wasted skill (well, maybe is debatable bringing some of them to the max level but everything is useful in the game). The only thing is the fact that you will not get the best conversation options if you do not invest at least in two * Arse skills.

I would say all of the skills are great to have, best to divy them up amongst your party. I recommend looking at a guide so you know what companions you will want to have and design the skills for your starting 4 rangers around that.

Here are some general advices, keep in mind that I’m not playing the game at the highest difficulty level, and I’m still far from the end game.

_the surgeon and field medic skills should go on a ranger far from the front, like a sniper
_the leadership skill should go on a ranger using mid-range weapons such as assault rifles
_the shotgun skill should go with a secondary weapon useful when cone damage is a bad idea ; hand-to-hand, bladed weapons or blunt weapons would be good candidate
_the energy weapon skill should go with a secondary weapon skill useful when fighting foes with a low armour rating ; sub-machine-gun would be a good candidate
_dialogue skills and toaster repair skill can give nice rewards, but if you play without them there’s no risk to get stuck because of it
_don’t focus on more than three or four skills on a single ranger, or you’ll have difficulties reaching high level in them
_don’t give multiple rangers the same weapons skills or they might fight over ammunition (well, two rangers sharing the same kind of ammunition is okay, more could become a pain)

I use different ways of playing that are also quite successful. Doesn't mean it's not good advice, just, not "should" advice. I play on Ranger difficulty.

- I like my surgeon/field medic to be at the front lines, either a melee user or pistols, that way a lot of the healing they are doing is to themselves (saves APs) and to the other tank-types who are getting the most damage done to them because they're upfront.

-I don't have any rangers that have 2 weapon types and don't find the need (and it would waste a ton of skill points.)
Though, I definitely make sure to have a shotgun that has a small cone for tight areas. All of my rangers have 2 weapons equipped for the varying circumstances and AP dispersion.

-Be sure to crouch before battle, but I almost never crouch my surgeon/field medic or shotgun user so that they can more easily change their positions without using APs.

-I don't have much issue with my Energy Weapon user not doing decent enough damage against low AC opponents. Sure, it does a bit less damage, but I have no problems killing the opponents and would find it to be a waste of APs to invest in 2 weapons.

I also only have 6 rangers but I have all of the skills, you've just got to do a little planning/pre-thought about how you will make them.

You have no difficulties to keep your surgeon alive and conscious in such an exposed place?
I know I don’t feel confident enough to take such a risk ;)

Nope. I use Rose (with pistols) as a backup to my blunt user tank and Scotchmo with shotguns in the "front lines" and she does fine. She's 3rd in line to get attacked, sometimes 2nd, I usually keep my health of everyone above 50% at all times (cept the tank, who is comfy down to 15%.) Every once in a while I might have to disengage her from combat so she doesn't get killed, but not very often, and even then it is just 1 turn for 2 heals for her and she's back in it. I sometimes get a heal and 2 attacks off with her (2AP midnight special!)

Sounds fun to play!
I didn’t think about joinable NPC it when writing my advices list, so they were more advices for a "self-sufficient" four rangers team. Maybe most of or differences in build come from there?
(playing at Ranger difficulty too, on my first run)

About the energy weapon specialist taking a secondary weapon skill, it is not only because of the lower damage they do against lightly armoured foes, but because I’ve often run out of energy cells and found it quite a good way to not waste them. But know that I think of it I remember she doesn’t have any skill point in her secondary weapon.

Actually, I play similar, my surgeon and medic are the main thank and a front-line support (Ranger setting). I have a secondary healer too (Rose or the native girl from highpool, even the mutant girl, or any npc with 3 or more int).

Is more easy that the medic reach quickly the fallen, that lost one full round because the sniper was the doctor.

Wow, you truly love to fire blue/violet! I usually end the game with 100-200 bullets for the lasers, but then I use high survival skill for selecting which type of enemies to fight in the world :P

P.d. There is a secret skill in the game, when you apply it to your shotgun user... the things become scary for the bad guys (Worse if the PC has the blunt weapons skills too). Is a good tip for the first game, but you should try to explore a little more with at least a PC.

In my last play I had my main tank with Shotgun, blunt and heavy weapons skills, plus surgeon, hard ass, disarm traps and the one for breaking walls (Literally, though guy who know how to break bones and fix things to the hard way). Only the weapons skills were not maximized (And probably trap disarm), a good int score let these things to happens.