Saw this discussion on TGA, thought it was worth flagging up after coming across a few of the weirder interactions (like the odd Tzeentch combo of re-rolling all failed and all passed saves!) at the club.
The crux of it is the fact that you apply re-rolls BEFORE modifiers, including rend.
So if you've got a 4+ save with a re-roll, but you're getting hit by something with rend -1. You'd only re-roll the 1s, 2s and 3s, even though a 4 will still kill you.
Pretty sure I've not been playing like this wrong so far (I don't think I even read the rule!) and even though its pretty counter-intuitive, it actually clears a few things up.