Mamba Max with a 4s
 

Is a BEC needed to run a MM with a 4s setup, it will go in a Rustler VXL, looking at running a Neu 1509/1.5y

Yes sir.

yes. rx pack or BEC required.

Good luck on taming that beast!!!!

What will actually happen if using the MM on 4s WITH bec ?
I got a new in the box MM that I am about to sell and I need test the MM before selling it.
Would the MM survive a "wheels in the air" test with a Feigao 9XL on 4s and bec wire still connected to receiver ?

Yes, just don't use the steering servo at all if for a short time. It's the current draw coupled with a high differential voltage that heats them up.

I guess unplugging the steeringservo would make if "safe" when.
As I said I only need to test / arm the MM.
BTW I might be stupid here, can I test the MM with Feigao on 2S ? (I do use 2S in serial normally)

2s should be fine if you just want to see if the MM works.

Thanks !