I loved the 3rd one but I heard that, back in the day, this version was crash happy and that it didn't run well. Now, that was ten years ago so I assume that this is no longer the case?

I had this on disc back when it first came out. There was some effort by techie gurus on a CIV forum just to get the game to install, and then frequent visits to solve crashing issues. Not just me - those gurus were pretty busy back then, solving issues with CIV for loads of people across different operating systems.

I remember getting through the game eventually, but all those issues spoiled the enjoyment. CIV is the only game, out of all the others (C3, Pharaoh, Zeus, Emperor, Children of the Nile) that I've not replayed over the years. I just look at the box and shake my head sadly.

Having said all that, when I saw it here, I had a moment of whizzing around the room in glee. Disc free, and maybe, just maybe, all those problems sorted. I want to buy it now, but my head is telling me to wait a bit for others here to come along with the thumbs up that CIV is looking good!

I also own CIV on disc, but since I got the new computer and Win10, I'm afraid to install old games from discs since the copy protection on one game made me have to formatt and reinstall the whole os.
Anyway, back when, I never had any crashing issues with CIV, and since I now have rebought the disc-free version, I've only had one ctd and I've put in a few hours already.
The graphics is even better than I remember. I was fearing the game would feel dated, but have held up very well.

Caesar IV used SecuROM, not SafeDisc. SecuROM has no issues with Windows 10, so you believed the clickbait scattered across the Internet. Also, SafeDisc can be re-enabled manually.

I play on windows 10 and have experienced very few issues. The only significant glitch I can recall is at one point my cursor disappeared, but I fixed that by running the game in compatibility mode for windows xp sp3.

I enjoy the game, but I'm also finding it pretty easy. That could just be due to my long experience of playing city builders for some 20 odd years, though.

There are a lot of major improvements for C4 over C3. I love the redesigned walker system - citizens going to the market and buying things makes more sense than random, wandering market ladies. Similarly, it makes more sense that any house within a certain range of road tiles would have access to a service building, rather than wandering actors hopefully choosing to randomly walk by the house you want them to. This also makes more sense than the earlier (caesar 2) design where a building radiated access in all directions, regardless of whether a house had road-access to it.

The biggest criticism I have for it is the military aspect is outright terrible. Troops are painfully slow to react to orders. There don't appear to be any significant tactics involved. I've been annoyed by this aspect enough I find myself buying off raiders rather than bothering to fight them - which is unfortunate because I would welcome the action to break up the pace of the game a bit, but with the way it works for now... it just hasn't been very fun for me. Caesar 2 got the military aspect right. I've get to see any other builder that could match it.

I've heard some complaints about maps being too small; is it an issue and is there a mod or editor for this?

Maps start off small in the Kingdom campaign. But they get plenty big enough as you progress through the Republic and Empire campaigns.