It's a pain to go to settings < cellular data every time. And it may happen when I don't notice it (like when I'm not using it). If there was a 4G toggle...I believe they are suppose to switch to whichever connection is better, though can't imagine 1 bar of 4G is that much better than 4 bars of 3G - though I don't know the specifics.
Why not just turn 4G off when at that location? This would help save your battery.