4 or 5S in a D8 conversion?

[RC-Monster Mike]

5s doesn't necessarily mean more weight - a 4s 5000mah pack has the same energy and weight as a comparable 5s 4000mah pack - with an appropriate motor and gearing selection, the total performance will be the same as well - both systems motored and geared for the same performance will perform similarly and have similar runtime. The higher voltage setup should have a slight gain in efficiency with a lower current draw if all else is equal.