You should be able to use standard m3 washers to shim it up to the appropriate height. However 3-4 mm of shims is a lot. Are you not able to true your tires down?

If you are able to true tires, I would ask around your local track to see what a common diameters is, I run 42.5 f/r. if you run on full size foam you will traction roll and chunk your tires very easily and will not be much fun.