I currently have a droid X (that is 3g of course) and my screen broke. I have been looking around on Ebay and found some phones that I like spec-wise (faster processors, bigger screens, higher resolution, front facing cameras).

A few of the ones that I like are 4g LTE phones. now the 4g LTE is not a huge selling point to me, I am quite satisfied with the speeds I got on 3g (and most of the time I am connected to wifi anyways). The big reason I am looking at these phones is superior screen size, resolution, processor speed, etc. and the 4g LTE is just a little bonus in my eyes, nothing major.

My main question here, is that if I were to buy one of the 4g LTE phones, would I be required to pay extra per month for 4g LTE? or would my monthly cost remain the same as it is currently with my 3g droid X? and also, if I buy a 4g LTE phone, do I HAVE to use 4g with it? Would I have the option to buy the 4g LTE phone and still just get 3g service on it if I am required to pay extra per month?