In addition to the changes to the backstrap, recoil spring assembly and mag release, the Gen4s have the SF contours available on the earlier SF models, as well as a modified RTF (texured finish). The Gen4 is 'smaller' in the hand than the earlier Gen3.
Early Gen4s had ejector issues that caused them to get a bad rep, but the current ejector designs have been 100% for me.
The standard Glock polygonal rifled barrels have a propensity to lead up faster than groove-n-land designs, and will spike pressures faster as a result when leading gets to be bad. You can shoot cast ammo through your Glock, but I usually clean the barrel every 100 rounds if I shoot cast. You cannot 'shoot out the lead' by following a cast lead box with a FMJ box - you have to scrub the barrel. I use copper ChoreBoy strands wrapped around an oversize bronze brush, and it pushes the leading right out. If you shoot cast lead in a stock Glock, you have to stay ahead of the leading and it's hard to see it forming - this is why most folk simply suggest avoiding it.
The Gen3 and Gen4 factory barrels that I've seen in 9mm, 40S&W, and 357 Sig all offered good-to-excellent case head support. The 45 ACP barrel in my Model 21, on the other hand, did not. I also run an aftermarket (Storm Lake) barrel in my range Glock 21, since I don't clean it much at all and it sees cast bullets almost exclusively (and the stock barrel wasn't all that great for case head support).
The other features, such as the bigger mag release, adjustable back straps, different texture (and what feels to me like more rounded corners) of the grip frame, were really just throw-ins as they were changing the frame anyway to accommodate the dual spring.
Interesting. The slide had to change to make room for the larger recoil spring assembly, but I wasn't aware that the frame internal dimensions were changed as well.
The early Gen4 9mm models didn't work well with low powered, light weight rounds.
Striker fired pistols have to use slide mass and the recoil spring almost exclusively to control the timing of the unlocking during recoil, unlike hammer-firing guns that can use both hammer spring, recoil spring, and slide mass. That makes the striker fired guns a bit more sensitive to changes in ammo power and spring strength.
This is also why they seem to be overrepresented in KaBOOM! incidents - they unlock faster since they don't have the snap of the hammer banging them into battery and holding them there for a fraction of a second longer.