If I buy an AT&T iPhone6 full retail, cut down a working iPhone 4 SIM card to fit in the 6, will it work?
It will work and the monthly bill will be the same for any iphone. Depending on what iphone model you have you might not be able to connect to AT&T LTE unless they change your iphone data plan to an LTE one. What iphone model you have now? But the cutting the sim part is pretty tricky. Unless you're using a sim cutter tool. Doing it manually you can damage the sim and make it useless.