I've never seen a cell suddenly die and go open circuit, you shouldn't fear that.
What is something to fear is one cell going eroding too fast, deviating rapidly under load, especially in cold weather. You can monitor that if you want to continue using it, which I deem fairly safe for now, unless something changes. I wouldn't send that battery on long missions anymore, or missions over water.
What I would do is put a special red tag on the battery and use it less than the others, since it's higher risk. If you fly it, you'll need to monitor that cell. The voltage displayed in the upper right corner (if you have enabled the battery voltage to display) of the Go4 app is the voltage of your weakest cell, and in this case that would be cell #4. If it ever gets close to 3.2V at the end of a flight, I would retire it. Never fly this battery in super cold weather, as a marginal battery can decrease momentarily in voltage more rapidly than in pleasant weather. If the weakest cell ever hits 2.99V, the motors will shut down mid-flight, that's why you want to retire a battery with a huge deviation. Right now I would classify this as a mild deviation, one to watch going forward if you need to fly with it.
You might see an improvement if you deep cycled the battery, but generally speaking that doesn't work most the time.