I have 4 cats that all use the same litter box (except one who prefers the corner of my living room

) and since I live in an apt, the best (and only) place for the box is in the guest bathroom. It always smells in there. I change the litter at least once every two days and completely switch it out once a week, but it always smells. I use Scoop Away for multiple cats.
Does anyone know which plug in covers the smell and makes the room smell nice. I used one with oil in it once, but it didn't work very well.
Thanks, I don't know why I ever got cats!!!