Do external booster antennas help with 4G reception? I have a Sprint HTC-EVO 4G.  Today it's snowing, I'm in a downtown high-rise, and I'm only getting 3G.  Would a booster antenna help (either now, or in general)?
It's hard to say without knowing what specific model(s) you're thinking of, but by and large the antennas you're thinking of are probably designed for 3G/EVDO, in which case the answer is no. Sprint's WiMAX network operates on a completely different frequency than it's 3G network, so the antenna for one will not work for the other. The EVO itself has two antennas in it for this very reason. Basically, if it doesn't support the 2.5-2.6 GHz frequency range, then it won't work.

There are WiMAX repeaters/antennas out there, but I've never seen one from a major manufacturer (or Sprint/Clear). Someone over on Android Forums also managed to make his own WiMAX antenna.