In answer to Vilita's question about wheels, there are lots of possibilities for a 4" wheel less than 1" thick. There are lots of possibilities for wheels, period. If I was making a robot that needed wheels I didn't have in my shop, here's where I'd start:
Check the hardware store and see if they have any lawnmower or wagon wheels 4" in diameter less than 1" thick. Failing that, check the plumbing department and pick up some PVC or ABS pipe caps. They should be available in about that size. Cut back the flange part of the cap until you have the thickness you want. If you have a lathe, it's a ten minute job at the most. If not, you can do this on a drill press by first hacksawing off as much as possible, mounting it to an arbor, chucking the arbor up in the drill press, and finishing it off with hand files. Run slow and BE CAREFUL!
Check at the nearest Goodwill, Salvation Army, or equivalent thrift shop. If anything there has 4" wheels the right thickness, buy it, strip the wheels as part of your salvage job, and shelve your spares.
Check at the toy store and see if anything has wheels about that size. Likely suspects are small wagons and rolling toys for toddlers.
Make your own on a lathe. This sort of goes back to the PVC or ABS pipe cap idea, but takes it one step further to where you design and build it from scratch. Stranger things have happened. You never know. I made a series of 4" diameter parts on a Taig lathe. Similar work could've been done on a Sherline or Asian mini-lathe. A good setup for any of these will run you between $500 and $1000. It may seem like a lot for a set of wheels, but the lathe will still be cranking out parts long after that robot is shelved or stripped.
Design your own and have someone else make it. Strange though this sounds, it bears mentioning. See if there's a home shop machining club in your area and see if any of them is interested in taking on the job. Better yet, see if any of them are interested in robotics and you may have a teammate. (Better still, see if anyone's selling a lathe used. Then you get your own home shop and you can join their club!)
Failing that, price out the job at a local machine shop. Shop rates vary and some shops price jobs as a way of controlling workflow, so do shop around.