I believe a "32-bit" processor has a 32-bit address bus, and a 64-bit processor has a 64-bit address bus. As for the external data bus ... ever since the original Pentium, all cpu's have had 64-bit external data buses. So that means the 486DX was the last CPU to have a 32-bit external data bus.
Yes, if the address bus is 32-bits wide, then that means it can address 4gb of RAM.