The 5s will offer more power and runtimes, plus it will spin the 9xl closer to optimum max rpm. If the esc can handle 5s I would say go for it. There is quite a difference between the power and temps between 4s and 5s. Plus your gearing will put more load on the motor with 4s, assuming you want the same top speed with both combos.
I will add weight factor also!
For race i will go 4S and play with gearing because handling is more important.
I say that on 4S lightweight draw less avarage when on heavy 5S.
For bashing and day fun and some race 5S OK, for race 5S power usless on heavy model. If 4S and 5S weight similar and ESC handle 5S go for it
Now I personally working on my truggy to make it light as possible and one thing shave some weight on battery.
A 5s 4000 mah pack and a 4s 5000 have same total energy meaning runtime will be less with the 5s pack to compensate for the higher power output. The 9xl will also run hotter on 5s because it is making more power.
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy