Very true.. I reckon you can achieve pretty much the same degree of surround subtlety/complexity with 4 as 5.1 speakers, the main things obviously lacking with 4.0 are the LFE which can be used for low frequency effects as the name suggests (stuff like the absolute lowest notes on things like a church organ?) but perhaps more importantly allows for better bass management and room correction (let's face it many living rooms don't lend themselves to the most ideal speaker layout and/or seating arrangement) and apparently the scientific folk say you get all sorts of anomalies like bass nulls with multichannel that a sub can cure more easily than other room treatments.. and then there's the other "surround elephant" in the room, the opportunity with 5.1 for a discrete Centre channel, the presence of which some love and others hate.. that said it's amazing the phantom imaging some old Quads have you'd really think there was a Centre channel on some of the mixes.. although thinking back to the room itself your sweet spot is more limited with Quad than 5.1.. It's been suggested a lot of the Columbia & associated label Quads were mixed with the limitations of SQ in mind and one of the things SQ could do pretty well was the 4-corner type of multichannel mix, I don't know why some of the Warner mixes are of the 4-corner type though when they were discrete all the way with CD4/Q8/Q4..
