My husband's capbadge is the only part of the Army that is expanding at the moment but the operational tour rate is appalling and the there are several people at his rank that are double and (in one case that we know of) triple-hatted.
4yr postings would affect promotion. For example: to pink, a major needs to have been seven years in rank with four reports; two at staff and two at command with the second of each type recommending promotion.
If each posting were 4yrs long, it would take eight years in rank before promotion becomes possible. Given how few promote first look, this would effectively hold people back.