This is hijacking somewhat, but as those 4 knots I quoted (without caring much about the actual value) don't seem to have many believers (and I confess I am in doubt myself) I did some research.
In France (and in other countries with CMAS) there are competitions of "scuba swimming" in a pool. The French name of these (so you can google if you like) is "nage avec palmes - immersion".
Competitors swim underwater (if they surface or touch the walls or bottom of the pool they are disqualified), usually wearing only a swimsuit, a mask, and long, powerful fins (no BCD), while holding their tank of air (no Nitrox) in front of them, with the bottom of the tank pointing forwards, to be as streamlined as possible. They breathe from a normal regulator like any scuba diver.
There seem to have been changes in the rules since the start (eg size of the tanks) but I'll give here the values of the French records in 1992. I found them in Guy Poulet and Robert Barincou's book,
La Plongée, Denoël, Paris, 1997. I checked also with Google on the site of the French Federation (FFESSM) just to find that nowadays competitors do better than in 1992, but with Poulet's values I have the size of the tanks used, without having to search any longer ; and also people used somewhat less specialized fins in 1992.
The distances are 100, 400 and 800 meters. The tanks used in 1992 were at least 0.7 liters (100 m), 3 liters (400 m), and 7 liters (800 m).
French records were not as good as world records

and were :
100 meters : 38" (hence 5.1 knots ; current French record is 34", hence 5.7 knots)
800 meters : 6' 34" (hence almost 4 knots ; current French record 6' 00", 4.3 knots)
I admit there are some differences between their setup and the setup of a regular scuba diver, even a very streamlined one (no drysuit, no bulky BCD, no stages !) using the same kind of powerful fins (much longer than those tested by
Diver magazine, BTW). On the other hand, 800 meters (and even 100 meters) is a long distance. Usually one doesn't fin against strong current for long, unless one wants to get overexerted. I'd say that, against the current, short "bursts" of finning, 10 or 20 seconds, maximum 30 seconds, between two shelters/places of rest, are more likely than minutes.
So maybe my 4 knots for a short time were not so absurd, after all.
Just food for thought (for me as well

)
End of hijacking.