First: Do the measurement at the spar with the other wing panel flat on the table. The measurement isn't too critical, so if it says 3 3/4" you're fine 1/4" either way.

Second: The rib that is off the table should also be relatively flat. (i.e. 3 3/4" from the table measured at the front OR back) If you have a 1 3/4" difference, it sounds like your wing is seriously warped unless I'm reading you incorrectly.