there is a dif between the gen 3 and gen 4 27, mostly being in the gen 4 mags are ambidextrous. they can be released from either side of the pistol, gen 3 will not work in a g4. long as your mags are all gen 3 or lower not an issue, will need a g4 mag being a g22 or what ever to work, in a g4 glock.