Hey, Tim. Did you end up finding one? Our DMS CNC Routers are really solid for really sensitive materials, but we don't tend to have many of them used for sale. : / However, if you haven't checked it out, you should see if Freedom Machine Tool Routers could do what you're looking for: www.FreedomCNC.com.  While they only come in 3 axis, are really industrial for their size and might be in the same cost range as something used. 

Best of luck finding a good fit. 

Have you looked at some of the DMS CNC Router options? www.DMSCNCRouters.com But if you're looking for something smaller for hobby work, Freedom Machine Tool is a strong contender - especially competitive in the 4'x4' size. And they give instant online quotes at www.FreedomCNC.com. 

Happy machining.