It will depend on the processor. For example not all address lines might be available from the chip so the physical addressable memory will be less than 4GB. Alternatively the processor may support paged memory and thus theoretically there is no upper limit to the amount of physical ram that could be supported. For example I believe some 32-bit Intel CPUs supported 36-bit physical addressing.
on its face the answer as others have said is 2^32 = 4294967296 but often, a memory management unit (MMU) is present that allows this to vary. A good overview of the MMU is at: https://en.wikipedia.org/wiki/Memory_management_unit
