Sounds quite promising overall actually. Unfortunately there isn't much you can do if you don't have something that can count amp hours like the BCM Gauge or OBDIIC&C Gauge. But your estimates are probably close enough.
The car is extremely self compensating - but you said the assist bars remained full, which signals 6.5kW of assist(or more, in 2nd and 3rd gears). 6.5kW of assist with a healthy battery is roughly 45-50A. So 45-50A * 4 minutes is 3.2 - 3.3Ah.
3.2 - 3.3Ah is........*drumroll*
About 80% of capacity.
You would expect 3.9Ah from a brand new 6.5Ah pack.
Still quite impressive, and is in line with what I've read regarding storing Nickel batteries. The battery actually probably remained surprisingly cool since it wasn't in use - the insulation of the IPU works both ways. It takes a long time for the battery to change in temperature even when the cabin is 120+.
They lose 1 - 2% of their capacity per year when in storage at room temp, but it shoots up dramatically at temperatures above that. I would bet this battery actually averaged about room temperature, since it seems consistent - about 1.7%/year average?