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.
The answer is yes. And although you didnt ask here is some side info : Kickers do not matter with Quads in Omaha. When you hold quads you can only loose to a straight / royal flush or higher quads. I think the odds of losing with Quads is around 1 in 56,000