4G in a 3G world (Australia) can I use?

[inquisitor]

Your mytouch 4G supports GSM (2G) at 850, 900, 1800 and 1900 MHz and UMTS (3G) at 1700/2100 (AWS) and 2100 MHz.
All three Australian network operators (Vodafone, Telstra, Optus) run their GSM (2G) networks at 900 and 1800 MHz, so you will definitley have 2G coverage, which will give you voice and smallband data service.
However for UMTS (3G) Australian operators primarily use the 850 MHz (Telstra) or the 900 MHz band (Vodafone and Optus) while the 2100 MHz band is only used here and there to increase capacity in urban areas. So except for the centers of bigger cities don't expect any 3G coverage with your mytouch 4G.