To really gain the benefits of a 6s pack (more efficient, cooler, longer life), you need to go to a lower kv motor. Also, like others said, it isn't a fair comparison to take a 6s 5000 mah pack and compare it to a 4s 5000 mah pack. The 6s pack in that example straight up has 50% more energy (and weighs 50% more as well).

If you can fit a 6s 5000 mah pack into the battery tray, then a 4s 7500 mah pack should fit as well. The difference between the two will be within 5% for run time and durability if you don't swap the motor and just rely on using less throttle/epa adjustments to reduce the power with the 6s pack.

When you use the EPA to limit the power, you end up with more switching losses. At 2/3 throttle (to roughly mimic 4s power on a 6s pack) the FET's are switching on and off thousands of times more per revolution than if you were actually at full throttle with a 4s pack. The switching losses eat up any benefit gained by going to the higher voltage.