Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
But, since ram is cheap, go ahead. If you should have a problem, you will have 8gb instead of 4, and that is usually plenty.
One note, running 4 sticks may require you to up the voltage a notch.
Also, split the channels, 4+2gb on each.