Is 4S+3S in serie possible?

[BrianG]

Yes, you can run any series arrangement you want. You could use 4s+3s, 3s+2s+2s, 1s+1s+1s+1s+1s+1s+1s, etc. It doesn't matter.

However, the MMM is designed for 6s max. 6s is 22.2v nominal, but 25.2 fresh off the charger. 7s is 25.9v nominal, but 29.4v fresh off the charger, which is too high for the MMM. That ESC has a device to absorb brief voltage spikes above around 28v (IIRC), but will blow on 7s.