I see that there are 3 replaceable back straps for the Gen 4 glock.

1st question is will the smallest back strap make the grip smaller than a Gen 3 glock?

second will this modification Gen 4 be on G20 and G21 pistols?

Word is that the small backstrap will be like an SF frame, so yes, it is supposed to be a bit smaller than a gen3 frame.

As far as the G20/21, time will tell. Personally I don't see a point. They already make an SF and the standard. Why would they make a gen4 G20/21? If they did, then I'd assume the small backstrap would be equivelent to the current SF, then add ons would make the grip like a standard 20/21, then what? Have another backstrap even larger than that? I have never ever heard anyone, not even some big guys I know who have gorilla hands, say that the standard 20/21 grip is "too small" for them.

that makes sense.

but I would think it would make even more sense, as long as you were developing replaceable back straps, would be to make one extra small, for women, like my wife who have hands the size of a 10 year old girl.

and then just make grip sizes up from there.

My understanding is that there are only 2 interchangeable backstraps.

The one piece stock frame is the one shown in the picture.

then you can ADD ON the additional back straps (only one fits at a time) over the frame.

Stock frame/no add on back strap - SF size
Use the first backstrap - current Gen 3 size
Use the second backstrap - larger than current Gen 3 size

Maybe Lone wolf/pearce or other after market company's will make a "straight" back strap since alot of folks that do glock grip mods to the other gen. models like to straighten it out to be more like a 1911 style feel.

Just guessing.

Robar makes frames and actually complete pistols now that have interchangeable backstraps, with a straight option as you describe.

But..............they're steel and aluminum.