Personally I think they've been getting easier ever since GTA 3, which I think is the hardest of the series. Going back to GTA 3 is hard because of all the advancements they made in Vice City, and San Andreas that made missions easier. One thing that irritated me about GTA 4 was the missions were more linear, in GTA III, Vice City, and San Andreas you could get inventive and finish a mission - couldn't really do that a whole lot in GTA 4. I don't think I ever played a mission the same way twice in Vice City or San Andreas.
That's true, one of the strengths of GTA3 was the ability to attempt different strategies when replaying missions, but the series has been moving away from that and has made use of triggered events rather than random ones. There are several missions in GTA4 where you have to chase a car and you can't shoot or ram it off the road, and if you even get close to it the car will accelerate at impossible speeds to stay ahead of you. That detracts from the feeling that anything can happen while playing, and it prevents the sort of unique events that made playing GTA3 so memorable.