If you intend to use a shared memory (SMPI) system then why not use OpenMP. Although message passing (MPI)will work on this system there is no advantage on the quad core system(other than having multiple virtual address spaces) and you have the disadvantage of the message passing overhead.
If you want to get in to MPI as opposed to OpenMP or other threadding environments then perhaps you should consider using two dual core systems or four single core systems. This will give you a better feel for MPI. With two dual core you could also test out a blend of OpenMP and MPI.