Since there are 2 M*A*S*H timelines, one where Potter comes in late summer 1952 and one where he comes in late 1950, it's possible that in the 1950 universe, Hawkeye does report Frank for being AWOL. This explains how we so quickly get Winchester there for the start of 1951. This means seasons 4 & 5 must be squeezed into the time from Sept. 1952 to about Apr. 1953 to give time for Charles to be there for a few months of episodes and then for Radar to leave so Klinger can be in his job for a month or so before the July peace. But, it can be done by juggling episodes.