From what I've read around the interwebs, If you can put it on, you cannot reliably do so for along period of time. When I was looking around for just a slide to purchase for my G23G4, there was a guy that was running a Gen 4 upper on a Gen 3 frame and because of the dual spring Recoil Spring, he was getting cracks in the gen 3 frame (or it could have been a Gen 3 upper on a Gen 4 frame- old age- can't remember now). I can't find the link now otherwise I would verify it for you.
AFAIR, he was shooting fine, but when he broke open the gun, the RSA was bashing against the inside of the frame. Other posters on the web have stated that they haven't been able to get the Gen 4 slide to even slide on to the Gen 3 frame. So there seems to be a bit of conflicting info on the web. However, there are a lot of folks who have done side by side comparisons and it LOOKS to me as if you cannot do it due to the differences in the slide and frame.
referencing another forum:
http://www.northwestfirearms.com/general-firearm-discussion/51393-glock-gen3-gen4-observations.html The OP talks about his personal experience trying to swap G3 and G4 parts.
not my picture:
http://www.texasguntalk.com/forums/pics/gen4/4.jpg Gen 3 slide vs Gen 4 slides
From what I understand, if the G3 gun had a Dual Spring RSA already, the slides are interchangeable.
You can do a G3 Upper on a G4 lower.
Not my picture:
http://i4.photobucket.com/albums/y148/MrVvrroomm/Glock Gen4/IMG_1532.jpg
Easiest thing to do is get a "complete" slide from Glockmeister/Glockstore or gunbroker that's compatible with your GEN Frame.
HTH
D
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