There is no Field Test Mode in iPhone 4!
For iPhone 4 users, the best way to check the signal level status is using the traditional *3001#12345#* code to enter the engineering mode and check the received signal level. However, the code doesn't work. Maybe Apple changed the activation code or it is completely disabled to avoid customer get the acutal received signal strength.
If you know the magic code, please do share it here. Thx!