only if you take a stereo amp and bridge it then you will get full 2 ohm power to a single 4 ohm speaker. remember when an amp is bridged to mono, it sees half the actual load.
if you have two single coil 4 ohm subs, wire tehm in parallel to a mono (one-channel) amp. you'll get a 2 ohm load, with power divided between the subs.