there may be problems with leaveing it in:
1. if you only have 2 RAM slots, obviously you can't have 4 sticks in so you can't leave them in.
2. The RAM you have bought and the other may not be matched. Even if both are the same frequency, latency, everything, they may not work. If you get 4 sticks of the same brand then they will, but still not neccesarily. My 8 gig of RAM for example i got from 2 packs, 2 of the same pack, so when i got it i got 2 packs of 2x2 GB on offer, so £140ish for £70 . Before that i had 2GB of RAM. The stuff i had when i very first got it was corrupt, so Corsair said send it in, we'll have a look. While i was having all of this I bought a cheap 2GB kit for about £20. When Corsair sent me out a new set, even though timings and everything were the same, it wouldn't boot with all 4. It wasn't the bios, that was the latest one so couldn't flash it to a newer one, both were in fine, there wasn't any dust on the connections, all the power settings were right, they just weren't matched so wouldn't work. Some will work together even though they aren't the same make from the same brand, but in general, 99 times out of 100, Different brands and makes of RAM won't work together