a good trick is to search the database for fleet numbers around 4xs
4XR is N9629H
4XW is N964TW
That combined with the fleet info at planespotters.net shows this order:
Which is off by one (maybe a TWA757 occupies a spot?) Anyway, there's a good chance it's one of those 3 in the middle (presumably N961TW since S comes after R). Maybe search the engine for pictures of these planes.