The Mamba Max is only rated for 3S -

http://www.castlecreations.com/products/mamba_max.html


You may get it to arm on 4S by disabling the onboard BEC and using an external, but that's hit or miss based on feedback from people trying it with the newer ones. The older ones were pretty consistent about arming and running 4S with the BEC disabled, that's the way I ran my G2R for a while.