A Gen3 G20 slide will fit on a Gen4 G20 frame, but MUST use a Gen3 Recoil Spring Assembly*. The Gen4 G20 RSA will not fit a Gen3 slide. Also, there will be open space between the muzzle end of the Gen3 G20 slide and the Gen4 G20 frame dustcover that is OK but unsightly.
* There is no Glock part called a "guide rod".
Welcome to the General Glocking Forum. FWIW, generally it is better to start a new thread than resurrect an old thread dead for 2.5 years.