If you're going to go through the trouble and expense to build a 4x8 machine, do it right, even if it means spending an extra hundred or three. I'd go with Bishop Wisecarver dualV wheels and guides. I'd guess that a set of 8 footers will cost about $500. You'll spend at least $200-$300 on what you're describing, and it won't work nearly as well and will be a lot more work.
The reason I went the conduit and skate bearing route was cost, nothing more. A 4x8 router should be fast, strong, and durable. I'd go a different route, imo.
