Well… Its not that simple!
First a Sapphire display is not as easy to see as it has a bit more refraction.
As far as being stronger not really! It is easier to fracture! Just less likely to get scratched.
I would strongly recommend you stick with the Ion glass. When I was fixing Apple Watches (series 0, 1 & 2) I saw more broken Sapphire displays than the Ion. I had one customer who in less than two weeks broken his display again dropping in on to a concrete floor. The third display he went with the Ion glass ;-}
iFixit currently does not sell the Series 4 display, you’ll need to try to locate a source for the display. You will need to transfer the NFC chip to your new display. Otherwise you’ll need to go to Apple to get your watch fixed.