Hm. @Chuck Sartor is pretty much an expert on these things. But I think the 4-wheel front truck you need is the one Lionel Fundimensions used on Great Northern #3100 in the FARR set, circa 1981. This is a very short wheelbase front truck that, as far as I know was only used on that loco.
I've personally never seen a 2046-style front truck fitted to the Berkshre. Not saying that it's impossible, but I wonder how it would affect the minimum diameter for operation. Lionel Fundimensions had the 2046-style truck available from the Blue Comet, Crescent, etc. When they wanted to make the green 4-8-4, they must have had a good reason for making something brand new that never existed in the Postwar era.
