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Hi dsssb, Wow, you have choosen a hard project. Since the pipe doesn't have to be structural sound, as it doesn't support any weight, you might get by with packing a thinn wall tube with dry sand, then end cap both ends, then cold bend around the correct radius. The coreect radius would be approx. 1/3 of disired radius to allow for spring back. The above is just a SWAG as I've never tried to accomplish such a task on larger pipe. I did bend some 1/2"emt to a large radius,(9ft), using the above method other than adding the sand. The sand would keep wrinkles from forming on the pipe surface as the pipe is bent. Maybe you could find a concrete round building with the approx correct radius to use as a mold. I'm sure it will take several tons of pressure even to bend the thinest gauge of 4" pipe. Another old method of working the pipe to a radius is to make a patteren with a finished radius on one side. Then use heat with a air water cooler to shirnk small thinn heat lines on the radius throat then cool with the air water thus slightly shrinking the pipe to a radius. I've used this method to bend a radius on I-beam with good results but I have my doubts if it will work well on pipe as pipe needs to stretch on the heal and shrink on the throat to bend and why a bending die is used. I worked with a pipe fitter from Venezuela that told me all they used to bend heavy wall pipe was the packed sand method and a large amount of heat and lots of pressure against another pipe. Another thought, try on a smaller pipe diameter, 1-1/4" ?, and weld a bead on the throat side of the bend then cool with water and see what you get. It may very well shrink enough for a large radius bend and still look good when done. If it works there then move on to the 4" pipe. T_Bone
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