I'd like to plant 4 American persimmon cultivars in a single hole, spaced at 6" apart with the intention of having them naturally fuse (inosculate) and form a single tree. Would this work, would it cause the tree to grow wider then tall - in comparison to a single specimen growing under natural conditions?

Personally, I don't think it's such a good idea unless you are very cramped for room and you want to try to grow different varieties on separate trees in a very small area. This subject (planting multiple trees in the same hole) has been covered numerous times in the Fruit and Orchards Forum. I'd recommend a search of that forum for lots of opinions and information about this practice. Generally, the problems seem to outweigh any advantage except maybe for the limited space situation, and, even then, it questionable. There may be advantages of this method over grafting different varieties onto the same tree, especially with persimmons, where you have limited space.