I‘ve posted somewhere - either upthread or on another thread - exactly why 4 bed couchettes wouldn’t be viable. The most you could get into the floorspace of a Mk5 sleeper coach, assuming you want some shared toilets at one end, is 8 x 4 bed cabins, ie 32 beds. This compares to 20 beds in the current layout, split between 4 x classic and 6 x Club. On the Highlander, that one coach will sell for a total of something in the region of £3,500-£4,000 in summer, depending on occupancy, and there will be ancilliary income offmost of them to in the club car. Therefore to make the same money (and igniring the fact that the operation incurs a cost per assenger) each of those 4 bed rooms will have to sell for something like £500. Now you might say “£125 each, great”, but you‘d have to be sure that every bed would sell in everyroom, and whilst that might happen in summer, it certainly wouldn‘t in winter.
The fact of the matter is that in this country, most people don‘t want to share with a stranger. I know some people are ok with it, but it is very much the minority, as CS’s research showed.