IIRC 4 is not considered an improvement over 5 actually. At least not in terms of gameplay. 4 has been likened to a "licensed romhack" aesthetically, but gameplay wise it's actually a pretty well paced and balanced game with solid level design. 5 on the other hand feels incredibly rushed with a lot of lazy stage designs (I frequently hear complaints about them being too wide open and sparse) and a slide that was kind of awkward to use. A notable speedrunner said 5 is the worst in the series, and I know at least several 1cc fanatic arcade players I've talked to about the series concurred that 5 is pretty poor while 4 is actually surprisingly decent. Of course that's by Metal Slug standards, which is really damn high. Honestly, I'd rather be playing the worst Metal Slug than some of the best Mega Man games. As for more detail on the first 3: 1: Some consider this the best in the series in terms of design. However some disagree because it's also the easiest due to the fact that you can have a slug (which takes multiple hits) throughout almost all the most difficult parts of the game. 2/X: Slightly more slugless action than X2, and as a result generally a fair bit harder, though not as much as 3. I personally lean towards this one as being the best. 3: Very divisive. You either love it for it's EXTREME amount of content and fairly well designed action or you hate it for having a weird difficulty curve that ramps up the trial and error from the second stage boss onward and has a final stage that's nearly 20 minutes long by it's own (the pacing for an arcade game feels kinda off in this one).