Since 4G is so new, you won't find much selection in 4G handsets proper, not yet anyway. Also- are you certain you meant 4G *GSM*? FWIW, GSM is not a 4G technology.
Let's be more specific though. Are you with ATT or T-Mobile? Knowing this information will tell me which handsets you can use since they will have specific frequency bands for 3G/4G service on each carrier.
Ideally you should have used the 30 day trial period to test out the new phone. Can't your new employer find you a way around the upgrade limitations(?)