It's a strange group (to me), but I don't see why it wouldn't work well. Once your team is geared up you may find that the DoTs from the Warlock aren't very useful (mobs die too fast). They could shadow bolt though. On a boss fight the CoE would be very helpful.

Glyph of Holy Light would cover a gap in the holy paladin's healing arsenal with AoE heals, and the Shadow Priest can backup heal if the Paladin gets MC'd/dies/etc.

It should work. If you are going to do a prot warrior, personally I'd probably go with a tauren to get the extra aoe stomp attack.