Answer
The energy is being supplied by the battery.
Work Step by Step
In part (a) and part (b), we found that $i_1 = 7.50~A$ and this current is to the left.
In part (c) and part (d), we found that $i_2 = 10.0~A$ and this current is to the left.
The total current through battery 4 is $17.5~A$ and the direction of this current is up.
We can find the rate at which energy is being transferred in battery 4:
$P = i~\mathscr{E}_4$
$P = (17.5~A)(5.00~V)$
$P = 87.5~W$
In battery 4, energy is being transferred at a rate of $~~87.5~W$
Since the current in battery 4 is in the same direction as the emf in battery 4, the energy is being supplied by the battery.
