I just finished a 4x4 build myself. I used steel tubes and a 12"x.5" aluminum extrusion for all the parts. I ordered all the linear bearings and ballscrews from china. The motor controller is a g540 that came in a kit with the steppers and powersupply.
All this and screws, wires, & switches for about $3000 usd.
I would build again in a heartbeat. The router has been extremely reliable, fast, and powerful. Building was the only way to get a router of the required size for our budget.