Any COOP, Migros or Spar will have one. They cost either one or two chf and vary in size between spacious and compact, not only can you put your shopping in them, there is generally a child tranportation compartment/basket just in front of the handle. The drawback is that they are hard to manoevre on and off buses.
Seriously, I have no idea what a four wheeled shopping cart is, other than a trolly. I got my hearts desire (a trolly on two wheels) from the broki for 3chf. My life is complete