I've never had a problem getting an extra hour or two, though 4:00 would be pushing it. I try to avoid negotiating in the following way, which seems to be very effective:

Me (on the phone): "Can I get a late checkout tomorrow?"
Hotel employee: "How late do you need?" [or want]
Me: "How late can you give me?"
Hotel employee: "Would 2:00 be late enough?"
Me: "Yeah, that's fine. Thanks." [If it's not fine, then we proceed, but I never make the first offer.]

By the way, if you call down from your room, they can see instantly that you're Platinum, which can't hurt.

Bruce