Welcome to TBN. You'll find plenty of information and hopefully some good advice here.
4x4 on a tractor the size of an MX5100 is the real deal if you will be doing loader work. Here's why. You've got a tractor that weighs something in the 6,000 pound range, including the loader. Maybe a little more depending on what kind of implement you have on the back. And you have a loader that can lift roughly 2,000 pounds, or 1/3 the weight of the tractor itself, sticking several feet out in front of the front wheels.
That throws the balance to the front and off of the back wheels, where a 2wd tractor has its traction. Now you can start hanging lots of weight on the back to bring it back in balance, but the overall tractor is getting even heavier and harder to handle. And you still have a bunch of weight sitting on the front axle doing nothing to add traction.
With 4wd, all that weight, both front and back will be carried by a drive axle.
Many people say a 4wd tractor will be about 20% more effective than a 2wd one of the same size and power. I don't know if that's right or not, but I can absolutely tell you my tractor will do twice as much loader work running in 4x4 mode vs. 2wd mode. You dig harder, go better, stop better, climb and descend hills better, and have more control over the load being carried. And the 4x4 will go in wet or slick conditions where the 2wd will dig ruts and mud trying to get traction.
A 2wd tractor can do loader work, absolutely, but it's not even in the same league as a 4x4 of the same size. Also, it's a bit different story with bigger farm tractors (say, 100+ horsepower). They have so much weight that adding 4x4 isn't quite so essential.
Well, that's my opinion, anyhow. Hope it helps.