A benchtop dc power supply acts as an ideal $4\: A$ current source as long as its terminal voltage is below $10\: V.$ Beyond this point , it begins to behave as an ideal $10\: V$ voltage source for all load currents going down to $0\: A$. When connected to an ideal rheostat, find the load resistance value at which maximum power is transferred, and the corresponding load voltage and current.
1. Short, $\infty\:A, 10\: V$
2. Open, $4\: A, 0\: V$
3. $2.5\:\Omega, 4\: A, 10\: V$
4. $2.5\:\Omega, 4 \:A, 5\: V$