I dont think you will be able to find one.. Take a 4" TC end cap. drill a 1/2 hole in it and weld the barbed fitting from the inside. Im sure their is a few people that will do it for you on this forum. Ask the guy from stainlessbrewing.com
Other options: 4" x 2" triclamp reducer, 2" triclamp clamp and gasket, and 2" x hose barb. Or DIY from 4" ferrule, hose barb, and copper disc.
These might work for you!! http://www.gwkent.com/end-cap-reducer.html
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