IIRC the lookup table will use a 'Number/Long Integer' to populate the table, as it is not actually storing the information that you enter in the other tables. So this should not be a problem.
The question is why are you using a fourth table at all. If you have the data in three other tables, a form will quite happily display the data correctly without referring to another table.
As usual I will point you in the direction of
http://www.accessmonster.com/ where you may find the answer you want.
Regards
ZH