MW3 was an excellent composition of game engine and graphics for it's time. Sadly, too many hands got into the pot for the 4th in the series - the production delays and bickering between all the parties involved in it's production ended up getting MW4 pushed through by different people, and it just wasn't the same.
MechWarrior 3 and the addition pack, Pirate's Moon is still a benchmark for games of it's type. There were a few things about MW3 I didn't like, like not being able to have human teams versus AI, but that's a minute issue compared to the many plusses the game offers.
Just a small example of what MW3 does that MW4 does not, is the "inertia" effects of running a 90 ton Supernova at full speed and trying to turn hard. The controls get sluggish and you'll have to compensate for that until you drop your speed. Another option is the ability to move weapons around to various stations better configurations of weapons and options over what MW4 allows.
In MW3, I can outfit a Bushwacker to be a deadly adversary against even the heavies, but in 4, the Bushwacker is fodder for anything above it's weight class...
Maybe look around and see if you can find a copy of MW3 out there, you won't be disappointed.