Not 100% sure on this but I thought that the IP5 had a nano sim, whereas the Nexus 4 uses a micro sim. Unless you have an adapter, the sim from your iPhone will not work in your N4. Your plan will not change unless you tell AT&T to change it. I assume that unlimited LTE also includes unlimited HSPA+, 3G, and edge since LTE may not always be available. Assuming that you have the correct APN settings, your N4 should (if the sim is the right size) have no issues using AT&T's network, and it most likely will not change your plan.