how much are you expecting to make at each job?
read the instructions closely - best to complete 4c on both jobs if the part-time job is at least half the pay of the full time job. Otherwise, filling out line 4b for the fulltime job and leaving 4b blank for the part-time job is the way this is supposed to work...
these are 'withholdings' only - it is not tax. if the withholdings are too high compared to the tax calculated on your tax return, that is what creates the refund. But if you follow the W-4 instructions closely, the refund / tax owned will be minimal.
