how to calculate the current at the 4 brushless motor
It appears that the data sheet lists voltage, current and power data points for the input of the electronic speed controller (ESC). The data is based on the listed propellors. For 50% throttle, the current is 1.6 amps per motor and 6.4 amps total. At 100% throttle, the current would be 7.5 amps each and 30 amps total.

The current taken from the MPPT controller and the battery would depend on the battery's state of charge and the level of solar panel illumination. With high illumination and low battery charge, the MMPT controller would supply all of the motor current. With low illumination and high battery charge, the battery would supply all of the current. The transition may be difficult to characterize.

In the diagram presented in the question, M1 thru M4 actually each represent the combination of ESC, motor and propellor as shown below.