UDIMM is a sub set of DIMM. In other words. if you buy UDIMM, you are actually buying DIMM. However, if you buy DIMM, you may be buying UDIMM, or RDIMM....
So, your questions is quite meaningless.
For 4,1, you can use either UDIMM or RDIMM, but CANNOT mix them together. In general, single RDIMM can has bigger size (e.g. a single 16G RAM stick), which UDIMM may not have.
So, if you want to go for large size memory, RDIMM usually is a better choice. Otherwise, UDIMM is fine.