The W-4 is a lousy form.  It’s very confusing.  My recommendation is to always ignore the top half and fill out the bottom with 1 or 2 exemptions.  Most people prefer getting a refund.  The fewer exemptions you claim, the larger your withholding and consequently the larger your refund.

You can really get in trouble if you work a part-time job.  The withholding tables don’t work very well when you work more than one job or have other things going on with your 1040.  I generally tell people to fill out Single  (or married but withhold at the higher single rate) and 0 with any part-time job.  You may even want to go so far as to have extra withheld if you have a number of part-time jobs.  Just remember - if in doubt, claim FEWER exemptions, not more.