Do you mean you replaced the SIM card with a 4g one? Also make sure your phone is on the latest update, the froyo version doesn't support 4g. You need to be on either one of the GB versions, (new one just rolled out a few days ago, very cool), or a custom ROM that supports it.
BTW: there are two types of '4g', LTE and HSPA+. LTE is faster and most consider it real '4g'. HSPA+, which is what our phone has, is somewhat between 4g LTE and 3g. Your notification should read H+ with a check or up and down arrows beneath it. It won't actually say 4g. This might help
Difference Between HSPA+ and LTE | Difference Between