That would make sense to me - though I have not travelled this line (north of Ockley, anyhow) a look at the timetable suggests the congestion is from the Epsom/Sutton area northwards. In the Dorking area you'd probably only need at maximum two fasts and two slows an hour, even at peak, so no need for 4-tracks.
This is also presumably the reason why Dorking, which is a fair size, gets such a slow service to London compared to other towns of similar distance to London of similar size. There is no capacity to run the trains fast, so it has to stop at the majority of stations into London.
The Waterloo-Reading could also benefit from 4-tracks out to say Staines, that's similar to the Dorking route in the sense that places like Bracknell and Wokingham get a very slow service to London. Nonetheless, maybe with careful timetabling they could squeeze in more fast trains. From at least 1981 (perhaps earlier) to May 1985 there were eight trains an hour to Ascot in the peak, and four of those were limited stop. And that was with what looks like a frequent suburban service - maybe they were just very clever with the pathing, ensuring that fasts overtook slows in the Wandsworth Town-Putney-Barnes area (four-tracked).
Yes, that's the other one that comes to mind, and that is a genuine main line (while say Dorking and Bracknell are commuter routes with a lot of traffic which could do with 4-tracking in parts to allow faster mid-distance commuter services).
In contrast to the other examples though, it seems to be the suburban services, rather than the main line (which all or mostly go fast to Bromley South) which suffer, with the all-stations services on this route not as frequent as one would otherwise expect in suburban London.
And of course this route had to, at one time, handle Eurostar too. Not sure exactly how they managed... though I do remember Eurostar crawling thrrogh Penge, Beckenham and the rest before accelerating as it switched to the Charing Cross line...
Didn't some of the Reading Waterloo trains go via the Hounslow loop? I seem to remember using them between Waterloo and Feltham back then