IIRC, you use the Gen 4 back plate, and ahould be plug n pray.

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Not in this scenario, No you don't need the gen 4 back plate.

You only need the gen5 backplate to run on gen5 frame.

So you can run gen3-4 slides on gen5 frame, with a gen5 backplate. More precisely- the gen5 TMH and connector needs more space to enter into the backplate when installing the earlier slide.

 

So the question in thread is actually the reverse... Can I use a Gen 5 Slide on a Gen 4 Frame?

 

With new barrel or if you use a g19 locking block I believe it should work

 

My brain missed the 17, stuck on 19, I think the Gen5 17 may have a different locking block than the Gen4, so NO. Others can confirm.