That's weird, but whatever, police trade-ins are everywhere, so it's not like that makes Gen 4s unobtainable by any means.
At any rate, make no mistake, there was nothing actually functional nor structurally wrong with previous generations of .40cal Glocks, unless you count the unsupported chamber, which is easily remedied with an aftermarket barrel, and really isn't a problem unless you go full Bubba at the reloading bench by trying to stuff a 10mm Auto load into a .40 S&W case, but that's really not safe for any gun, not even if you plan on shooting it out of a particularly robust pistol like an H&K USP40 or a S&W 4006, let alone even a 10mm Pistol for that matter.
At most, you'll get some slightly bulged brass with full-power factory ammo, which can be a bit of a pain if you're a reloader, but again, buy pretty much any aftermarket barrel and that problem goes away because they basically all have full chamber support.