You might already know that HTC probably won’t provide Inspire 4G an official ICS update, that sounds bad for their users since they will stuck on previous Gingrebread update. And the good news is, zFr3eak a developer from xda-developers have successfully porting Ice Cream Sandwich to work with our android phone, he build this custom ROM for HTC Desire HD but some members reporting that this ROM also works on Inspire 4G too since both has identical hardware.
Although the firmware is still beta, but many major features have reported to worked well such as touchscreen, call, wifi, GPS, 3D/2D acceleration, accelerometer, bluetooth, and many more. But I think you should wait until the dev release the stable version if you want your camera work. Or if you prefer to running CM9 which based Android 4.0 you have wait much longer. In order to install a custom ROMs, for now you prepare your Inspire 4G by giving it a root access and S-OFF status. Since HTC Inspire and Desire HD has similar hardware, the DHD flashing method also work for this AT&T android phone, you can learn more on this post.
For those who don’t want to bother or have no time to manually flashing your smartphone, you can upgrade to HTC Vivid (AT&T) for only $165, the price is worth for Dual-core and 1 GB RAM and confirmed to get official Ice Cream Sandwich update from HTC.