All corporations produce new products ever so often whether they actually need to or not in order to grow the bottom line, increase stockholder equity and grow the company. They then attempt to make you want it with marketing that basically says you can't live without it. For me the Gen4's are an "evolution" and not a "revolution". But my Gen4 19 (flawless through > 2,000 rounds) is fast becoming my favorite pistol. So I guess that's what's up, at least partially, with the Gen 4s.
There are plenty of Gen3 guns where I shop but that may be stock backlog. I don't think Glock has committed to making every model and variant in Gen4 config. Time will tell if Glock stops Gen3 production, but they no longer produce Gen1 or Gen2.
My Gen4 19 was marked $30 higher than the Gen3 19 laying next to it. I was told the additional $30 was because the Gen 4 came with three magazines and the Gen 3 only two. All the Glocks I've ever bought came with two magazines with the exception of the Gen4 19.