Is it possible to start a game in Darkside? If possible, is it probably way too hard and would the story end up all disjointed? I haven't seriously started the game yet but I'm always down for a challenge and I've read that starting in Darkside would be a lot harder. I don't really want to do it if the story progression will be all jacked up however.

You can just walk directly out of the starting town, go into the pyramid and you'll be in Darkside.

That said, I'd really recommend starting with 4. If you start with 5 then you'll absolutely STEAMROLL 4 when you get to it, but if you start with 4 and then move on, 5 can still provide a decent challenge beyond the first town. This is because you level up MUCH more in 5 and the game is balanced for that, so being max level from 4 makes very little difference, while being max level from 5 makes a huge one. And you're going to want to do both games, if only because the full plot pretty much requires it.

Might and Magic V Darkside of Xeen was released and is playable as standalone game, so it's no problem playing it without part IV.
If you create characters in the Darkside Inn they start at level 5, so you have to be a little careful until you gained a few levels or get to the Inn there for creating new characters, but it's doable.

I recommend playing MM4 first since you can only reach level 20 there, for an additional challenge you can explore the two caves in the lava there before getting extra power from Darkside.
If you start in Darkside and travel to Clouds afterwards with all the XP gained the overall game will be even easier.

You can very easily play both games "at the same time", by which I mean, you can do some things in Clouds of Xeen and then head over to Darkside and do a few things, and keep switching whenever you want. But as others have said, the games were originally released separately and so it's completely possible to just do Clouds in its entirety first, and then tackle Darkside. Darkside was designed to be harder, but still possible to start with.

Because the GOG version has the two games merged, it's not possible to just start a new game in Darkside. But as sethsez pointed out, you can very easily walk there from the first town in Clouds. If you want to use a trick, you can take some characters to Darkside, sign in at the inn, and then make new characters there. Characters made in Darkside start at level 5 instead of level 1, because of the increased difficulty in Darkside. However, this is totally not necessary, as your level 1 characters will have plenty to do in Clouds.

I'd recommend just starting in Clouds and then switching over to Darkside whenever you feel like to try some quests over there too. You'll probably find that you'll spend most of the early game doing Clouds content since it's easier than the Darkside content, but you can definitely tackle the first 1/4 to 1/3 of Darkside while you're finishing up Clouds. Then the high-level endgame stuff will primarily be in Darkside. One cool thing about the games though is that there is some special content that's specifically designed to take you back and forth between the two games, like certain dungeons in Clouds that can only be unlocked with keys found in Darkside. I thought it was a cool touch.

Anyway, enjoy, they're really great games!

Yep!

1. Navigate to your install folder and rename "xeen.cc" to something else (i.e. "_xeen.cc") -- when you start the game next time it will just be Darkside, not WoX.

2. Start a new game, save, and exit.

3. Rename xeen.cc back to xeen.cc -- this will convert Darkside back to WoX. Now when you start WoX it will convert your save game from step 2 into a WoX game but keep your position the same (i.e. Castleview).

Have fun starting on Darkside!