When playing Omaha If the community cards have 3 nines come up and I hold the 4th nine can I claim four of a kind?
Yes. You must use exactly two cards from your hand to make your best 5-card poker hand, so if there were three 9's on the board your best hand would use the 9 and the highest ranked other card from your hand. For example, if you held KQ97 and the board were 999AJ, your best hand would use the 9 and K, for a best hand of 9999K. Of course, in this case the kicker is meaningless, since no other player can also have quad 9's.