Mick,
If we were talking about the time it takes to do 4 shots you would be right but in this case I'm only interested in the actual reload time.
The reload happens between firing the second and third shots.
Time taken to get all four shots away will include variables like how fast/slow the shooter is regarding getting on target and actually firing each shot etc.
These variables can alter the result.
The issue with O/U that always seems to crop up is the wide gape causing slower reloading than SxS.

Are you saying that a good shooter should be able to fire 4 well aimed shots with a SxS in 3-4 seconds or just complete the reload in that time?