Chances are you can add two Crucial sticks, and suffer no ill effects. You'd probably want to run Memtest86 to check for errors (I do this on every new machine anyway). As long as the sticks have the same basic specs you'll be fine.

But yeah, dropping one, and adding three identical would be ideal. But ideal isn't always needed.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs