Older 4g phones switch to 3g when they think the signal is stronger. Its stupid because 2 bars of 4g is stronger than 3 bars of 3g. Ive heard stuff about updates to fix that for the longest but havent seen any solid fixes. Thats why people with razr's, rezounds, droid 4, nexus, etc will have 4g with no issues while other phones stay in 3g for hours after the service is back up and running. You can try a power cycle and a sim card pull to see if it wakes it up.
Thats the plan after 4G in all markets.