I want to find an expression for \(I_5\).

I see that \(I_{S1} = I_4 - I_5\), but how can I proceed from here?

 

Find the Norton equivalent circuit for the terminal across R5 and then calculate I5.

 

I think I figured it out:

\(I_{S1}=I_4 - I_5\)

and \(-I_5 (R_1 + R_5) = I_4 R_4\)

Solving for I_5 gives

\(I_5 = \frac{-I_{S1} R_4}{R_4 + R_5 + R_1}\)