Hi - It's a crapshoot. Very often the same make/model RAM will not work together when not
purh in the same set. Sometimes it will work. If you do choose to do it, you mobo most
likely supports dual channel RAM, so be sure that the sticks purch (such as the 2x4g) at
the same time are in matching slots on the mobo, very often they are not next to each
other, ie slots 1&3 for one set and 2&4 for the other, your manual will tell you which.
Some times matching slots are diff color making it easier to select.
This assumes your OS supports more than 16g total RAM (ie Win home 7 prem supports 16g max)