Ok, upon reviewing the original post I realize that I mis-read it. Somehow I thought you wanted to go to a 4’ x 8’ bed. That’s a big jump. I’ll blame my error on being on vacation in Hawaii and the mai tais. Now that I’m back and in a more serious frame of mind here’s some real numbers that should help you.
I built a quick CAD model of the Y-rail and ran a FEA to analyze the displacement due to the load. I made some assumptions to get things done but they are the same across all configurations so at least the relative differences should be similar. So, assuming that the Journeyman X-50 rail with spindle is 50 lbs I put a 25 lb load on the center of the rail. Here’s the result for the original Y rail:
The displacement scale on the right shows that the standard WW will deflect about 0.0006 in. That’s really good for a hobby machine. Note the displacement is multiplied 2000 times for visibility.
Here’s what it looks like if we just use longer 35 mm tubes to get 48" of travel:
Now we have about 2 thousandths of displacement. That’s over a three fold increase. Now realistically 2 thousandths of an inch is not a lot and you’re unlikely to even see it, especially if you’re just doing a carving.
Here’s what it looks like if you use 50 mm rails for 48" of travel:
That brings us just about back to the original Journeyman with 0.00065 inches of displacement.
So if you can live with 0.002 inches of sag then just buy longer 35 mm rails and screws. Just a note but in my experience I’ve found that hollow rails are considerably more expensive than solid ones. I’m sure that OF is getting a deal by buying hundreds of feet at a time. You can use solid rails but you’ll lose the nice wires inside the rails. You may have to flip the motors to the other end and reverse their direction.
Unfortunately, going with 50 mm rails won’t be as easy as getting OF to sell you two more X-50 rails. You’d need to make new carriages for the Y-rails to mount eh X-rail to. So unless you have access to a milling machine you’re better off waiting for OF to offer a 48" x 48" machine and upgrade.