Cats are definitely a part of this site, and we're glad you found us.
i second the suggestion of an adjustable cart. we made a temporary cart -for Mimosa is was more of a PT tool - out of a kid's toy we bought at Walmart. put her belly on it and attached her with vetwrap. It worked fine for short periods.
we have a two legged cat here, TwoBit, born w/out hind legs who just walks around on her front legs and holds up her butt, sometimes 'bouncing' it along behind her. As your kitten finds ways to get around, keep a close eye on the legs, feet & tail for any drag-burns or wear. I remember a garment some people talked about that had front leg holes, then a long piece of fabric that went under the rest of the cat so they could pull themselves along without worrying about friction burns. anyone remember where to get these?
at 4 weeks it is a bit early to tell if kitten is eliminating on his/her own, do you have the momcat or is this a bottle kitten? you might want to read the information on expressing, just to be ready.
Welcome to Hpets!!!