So rattle on my gen4 is completely normal then? Like when I tilt it back and forth?
It is absolutely. Like I said above, though, you should look for Youtube videos, if necessary, to remove the striker on your Gen3 and clean the striker channel. Crap can build up in there and eventually start causing light primer strikes or a complete failure to fire. It comes from using too much lube on the slide. I forget which gun it was, but I took one of mine over to Glock and the armorer cleaned it thoroughly and told me to back down on the lube.
Glocks have a partially cocked striker, so there's no pressure holding it still. That's why it rattles. The rattle means it's moving freely in the striker channel, which is a very good thing!