I was pondering that same question a few years ago and just split the difference and went with #5s (no kidding - my logic was that the #5 would be the best of both worlds with a greater pellet count than 4s and more downrange energy than 6s).

In my opinion this was largely an academic exercise, as he turkeys didn't seem to have a preference - or they simply didn't care to express a preference - following the clean one-shot kills. )