With four DIMMs the BIOS will downclock all of the RAM to the default frequency of 1333 Mhz. --- assuming the DIMMs will actually function together as a non-matched set with out issues -- which is a big assumption. You won't be able to OC the RAM with four DIMMs installed.
If you want to run a lot of or large apps like CAD, etc. then more than 8 GB. RAM might be of value. If not then 8 GB. is plenty. OC'ing the RAM makes less than 1% diff in system performance in my testing.