I have been flying the HK Nano Tech 3300 4s batteries for about a year. Fully charged each cell has 4.19v. After each flight they go down to ~3.79v. I put them on storage mode to 3.86v.

Yesterday and today on the one battery when it got to the cut off blip the motor just started getting slower and slower.  Like there was not enough voltage to spin it back up to full rpm.
When I check the cells I had 3.69 on three cells and 3.59 on one cell and showing 9%.

It's it time to soak this battery in a bucket of water?

You probably hit the low voltage cut off. You can check the ESC settings and maybe go slightly lower than the default cut off voltage. If you give the battery a full charge after flying and the charger puts back more than 80% you can put that one in the practice box. What I do is set the timer for less time and skip as many level laps as I can so I get done the pattern before the time or battery runs out. It'll give you a little more use out of it but it's on it's last legs so watch it closely.

If you want to scrap it don't soak the battery, it doesn't work. Get a small automotive light bulb and hook it up to the battery for a few days then solder the + and - together and throw it in the regular battery box at the recycle center or take it to radio shack.

Hope that helps,
MM

What value is your low voltage cutoff?

Sounds like that battery is done. It's not holding voltage under load. For disposal Motorman does it the way I do so follow his instructions.