I have memories back in the 80s of the Fort William portion of the sleeper running with 2 sleeper coaches and the 2 day coaches.
The Aberdeen sleeper has ran with only 3 coaches a few times since the Mk5s came (have seen variations of 2 sleeper + 1 lounge, and 1 sleeper + 1 lounge + 1 seated).
My recollection is that in the late 1990s and early 2000s the Fort William portion usually consisted of 4 vehicles (2 sleepers, lounge and seated coach), and my impression (possibly incorrect) is that it's only fairly recently that they've taken to sending more sleepers to Fort William and fewer to Aberdeen.
For a few years in the late 1980s and early 1990s there was a Poole portion of the West Country to Scotland sleeper, and on the one occasion that I used it, it seemed a surprisingly short train.
This picture shows 4 vehicles.
I'm guessing that 4 vehicles might be the shortest sleeper formation that's been regularly scheduled (i.e. not including failures) to run over any significant distance (i.e. not including shunting moves) in the UK in recent decades, but maybe others know differently.
Not UK, but relating to that example it used to be very common in mainland Europe for there to be single-coach sleeper "trains" which would just get from A to B very slowly being shunted on and off various other trains, but would be a train in their own right with its own train number. They often spent a fair bit of time sat in sidings waiting for their "connection"
Still is pretty common, Bucharest-Istanbul and Moscow-Pyongyang are both only one through car attached to various trains enroute.
In 2007 I travelled from Ulan Ude (Russia) to Ulaan Baatar (Mongolia) on the Irkutsk-Ulaanbaatar train, i.e. not the through train from Moscow. If I remember rightly, much of the journey was as part of a long-ish train, but only two coaches made the journey from the last Russian station to the first Mongolian one, which was more than just a shunt (I can't remember exactly how long it took, but the Internet suggests about 40 minutes, which seems right). There were indeed long waits at both border stations. Online accounts suggest that sometimes only one coach crosses the border.