While i too think there's no technical reason why a new SIM is required for 4G, could it not be that (for whatever reason) some networks will require it?
im not knowledgeable in mobile phone networks and the technicalities of how they operate, but could it be possible that something on the SIM would need to enable the connectivity to the 4G part of the network?
didn't 3G require a new SIM (USIM as they were called)? If this suggestion that because 4G is just the hardware in the phone, why did 3G need a new SIM?