Hmmm 3 to 4 years eh. Well thats a little narrow but based on that premise I'd say 1919 to the 1923 time frame.

The division is hard enough to pinpoint as welters always seem to figure prominently in most great middle weight eras. I mean I suppose 1945 to 1955 give or take a year either way would be in the running if the era was a 10 year glimpse.

I think the better mid divisions were generally found pre weight class expansion.