How long are the 2" pipes, What process are you welding with,how good is the fit where the 2" mates into the 4", do you own a machine shop or have a good buddy with one who could make a V block tie down with clearance to weld around the pipe with out unclamping.
Heat input is your main reason for warping. Molten metal shrinks when it cools so the least amount of input filler metal to do the job will minimize the drawing effect. you might machine a 1" thick round circle to insert into the end of the pipe to keep it from deforming, drill and tap for 1" 8tpi and use a slide hammer to remove it. Every action has an equal and opposite reaction. you may have to take a torch and apply the same amount of heat opposite the weld to counter act the pull from cooling. Reply with some more particluars on how you are clamping the pipe and we will come up with something. good luck the Farmer