Even though you can read the individual books of the Quartet seperately, it is better if you read them in sequence.

Sinai Tapestry sets the scene for the rest of the sequence and introduces the main characters so you would be missing out on quite a few references if you read Jerusalem Poker first.

Sinai Tapestry is a lot of fun to read too, though Jerusalem Poker is probably more outrageous. Both are wonderful novels.