STUVA litterbox for 4 kitties - mine would probably prefer two separate chambers. STUVA does come in a single-width drawer, and you could get two. The trofasts have very low sides, though -- you'd want to use high-walled boxes so they didn't kick litter everywhere. Then you could put standard cabinetry on top. Love it!
Litter box cover from 2 IKEA side tables and fabric! This is a great DIY for cats, better than traditional covered boxes because it doesn't have plastic on the sides, which can trap odors and make some cats reluctant to use the box as often as they should.