How do I stop the 3g/4g radio from turning off?
If you see the 3G/4G symbol and the arrows are just grey, that just means that you have data connectivity but there is no send or receive activity at the moment.

I guess you made an false assumption: The data radio is only off (deactivated) if you don't see the the symbol at all. Otherwise, if the symbol is there and the arrows are grey, your radio is on, but there is no traffic happening at the moment.