For plain scrypt I had no problem with 4gb on any of my rigs

However for scrypt-n (e.g. vertcoin) my rigs with 4gb ram can only run 3 280x's. My rigs all refused to run more than 3x280x with 4gb ram, excess cards showed as dead with error message in cgminer. I upgraded my rigs to 8gb ram and then all of them were fine, so I concluded that for scrypt-N 4gb ram can only run 3 280x cards, whereas 8gb ram seems fine for 6 280x.