There is probably a formula somewhere that will tell you how much force is required to bend a 4' piece of 1/8" steel. I am guessing that it will be a lot. You might need to look at a hydraulic press design. I have seen some pretty nice homemade pan breaks that could handle a 4' width, but not 4' and 1/8" thick material. Anything is possible, but a pan break to bend that would have about a 1000 lbs of metal in it.