Don't use just any brand of crystals...
We thought we liked Target-brand Crystals b/c you could get a big jug of them for $9.95, which was nice b/c the Tidy Cats stuff and other Crystals are expensive. At first they were fine, but then (we have 2 litter boxes and with the crystals, we didn't have to change weekly - more like every two weeks or so), they had this weird smell to them (when it had only been a week or so) and the cats didn't want to use the box, even though we clear all the waste out of them 2-3 times per day. It was just odd.
We liked the Tidy Cats Crystals, but unless we're going to have a coupon every time, that's just too expensive for a household with 4 cats. We had the Tidy Cats Crystals Blend for a while too, and while we liked the fact that it was far less dusty than the regular clay Tidy Cats, it tracked all over the house more, and we couldn't flush it down the toilet like we like to do normally - oh, and if some got on wet floor, it would cake up. It was gross. At some point we may have to switch to something else, though, b/c I hate all the dust that Tidy Cats clay litter kicks up, though we don't have too many problems with "cement" on the bottom of our boxes (and we can't use liners b/c Leo would tear them up). With Tidy Cats clay, we usually change it every weekend and definitely clean it 2-3 times per day. Is that what you should do with 4 cats and 2 litterboxes, or can it go longer than that?
We normally don't have a smell in our house b/c we're so good about cleaning the litter, but if we're gone for a day or a little over a day, we can smell that we need to clean the litter when we walk in.