Chiming in here...the aftermarket part is absolutely incorrect. Gen4 has the most - it has Gen 3 + Gen 5 compatibility. There's only 5 standard parts from both camps that it can't use.

• Gen 3 frame parts fit in Gen 4 frames. Only parts not compatible is the mag catch (Gen4 uses the Gen5 mag catch) and polymer trigger housing itself (the spring, connector, trigger, and ejector will work). The Gen3 trigger housing pin is different, but still works in a Gen4. Gen 3 slide parts fit in Gen 4 slides. The only part not compatible is the recoil spring, but it can work with an adapter. Everything else from a Gen3 works in a Gen4, hence why some parts are referred to as Gen 3/4.
• You can run Gen 3 slides on a Gen 4, but you can't run Gen 4 slides on a Gen 3.
• Some Gen 5 frame parts fit in the Gen 4. The incompatible parts are the ambi slide stop lever, and coiled slide stop spring. The trigger housing is shaped the same, so while each individual part of a Gen 5 trigger is incompatible, the entire trigger unit can be dropped into a Gen4. The Gen5 Glock Performance trigger will work in a Gen 4...it just prefers a larger trigger shoe. The slide shape of the Gen 5 is identical to the Gen4, but uses its own Gen5 internals. Likewise, each individual part of the Gen 5 slide will not work in a Gen4 slide, but the entire completed slide can be run on a Gen4 frame.
• You can run a complete Gen 5 slide on a Gen 4 frame. You can't run a complete Gen 5 slide on a Gen 3 frame.
• Gen 5 G17/34 slides will work on a G17/34 Gen4 frame as long as the barrel and recoil spring are Gen4 compatible, and the internals match the slide (ie Gen5).

If you meant things like Gen3 and Gen5 magwells, and the Radian Ramjets, well then yes that part of the aftermarket is lacking for Gen4, but that's a small subset of things not compatible. The fact that gen4 will run Gen3 and Gen5 slides, already puts it ahead in parts.

Now in terms of the least desirable 17... I guess if that is personal preference. No front serrations, slide finish is weak, perhaps fingergrooves don't fit...all valid. I would put the moon cut Gen 5s in that list as well.