I have a new 19X. Love it, now I also have a Gen4 19 and a Gen4 17. Would it be possible to mount the 19 slide on the 17 frame and have the combination be reliable?

No. You can't simply swap slides.

The Gen 4 17 and 19 have different dimensions for the locking lug on the barrel and the locking block in the frame. They also have different dimensions regarding recoil spring interaction with the frame.

Even on the latest Gen 5s, where they went to one standard locking lug/block for simplication, they're still not compatible due to the differing frame dimensions. The Gen 5 G19 slide will physically attach to a Gen 5 G17 frame, but the slide still doesn't have enough travel to move sufficiently rearward to properly pick up cartridges and eject cases. It bottoms out too quickly.

To think of it another way, the G19X is a G19 frame with a longer grip/magwell, not a G17 frame with a shortened front dust cover.

Here's a photo I found showing the different lockup distances for the G17 (left) and G19 (right).

Thanks JSK. Thought it would be a good project.