How many mAh did you get into the battery when charging from 3,8 to 4,2 V? (/cell).
If you want another flight on the battery then try to get a high C
nanotech to replace your 20C Lightmax.
If you have not already tried the difference you will be surprised. I think you will get the feeling of a much bigger motor when you get a lot more out from your battery. This is really noticably when it is a little colder outside.
If you say that you are flying 6 minutes then you will fly at 10C average - that is 40A average on your 4000 battery.
I dont think so......