Right now I have a LG Optimus S and the phone was a reallly good phone before the update after it started having issues but I only got this phone because i wanted an android and i thought that if i got a 4G phone i would get charged more than the $10 Unlimited Data Fee but now im seeing I am wrong i can get a 4G phone with no extra charges I think?
So my first question is are there any other charges for having a 4G phone even though I already have an android smartphone?

My other question is if i were to get another 4G phone from a friend who is selling his and i want to connect it to Sprint to replace my current phone would it effect my account in any way? For example would my contract extend and would i still have insurance on my phone even though i changed it to another one?  Is their anythin that would change?

PLS HELP!