From what i understand from what i recently read on an IT forum yes, but it would be pointless since the 1600mhz would be clocked down to the slower 1333mhz. i could be wrong but either way i wouldnt recommend mixing ram speeds.
also 1333mhz ram is generally cheaper than 1600mhz.
133mhz is not an OC ram and 1600mhz is for OC but if you plan to OC your cpu dont think about mixing 1333 and 1600.
I would say either stick with just the 1600 if you plan to overclock your cpu or just get 1333mhz if you do not since it costs less.
They should work together fine.
The old kit is 1.6v as opposed to the new kit which is 1.5v.
The modo is defaulting to 1.5v anyways at 1333Mhz.
I don't see a problem.
Just put the new kit 4GB in the main/boot dimm.