Isn't illegal to do this yourself now? Not sure what the penalty is or how they'd even enforce it...but isn't that the case now? Like they will let you unlock if you're off contract, but if you're on contract...you're locked to their service...because they subsidized the phone you just got.
This should work. If not, they give you your money back.
http://cellunlocker.net/unlock-samsung-galaxy-s4.php