Best answer by Adzdalton
So your smartphone will use a whole bunch of parameters to determine which mobile tower and what frequency (3G/4G ect.) to latch onto.
It's not just signal strength but also the quality of the signal. I.e heavy traffic on a particular frequency in that area can slow the speed down considerably.
Usually your phone will choose the best frequency for you, if that means dropping to 3G but getting better quality transfer of data.
You can of course set your priority yourself to 4G and see if you notice any change in quality. As your on the move tho i would recommend keeping it on auto.
Hope that helps :)