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its been recommended not to use 4g toggles. tho there may have been an improvement to the stock toggle after the 3.7 update.
if you are using a toggle. turn the toggle on (whether it connects or not) no remove it from your home screen. then go into wireless setting and turn off 4g. now reboot your phone.
once your phone boots up go back into wireless settings and turn 4g on. stay there and watch it turn on and scan until it connects.
does it now connect? or does it turn off and disconnect? if it turns off on its own and does not continue to scan (or connect) then its a problem with your phone. software conflict of some sort. 4g should stay on and continuously scan if the phone is working properly.
if not, you may have an issue. factory reset might help as a last resort. are you rooted and using a custom rom?
the 4g issue is explained on this thread in section 1 Super Evo Battery Tips and Explanations **Updated 1-22-11** - xda-developers