calculate unknown source potential
I don't want to use the delta formula ...


That's unfortunate because that's the formula to use.

\${{4.28\text{V}-1.38\text{V}}\over{4.91\text{A}-0.67\text{A}}} \approx 684 \text{m}\Omega\$

\$4.28\text{V}+0.67\text{A}\cdot684\text{m}\Omega\approx4.74 V\$

\$1.38\text{V}+4.91\text{A}\cdot684\text{m}\Omega\approx4.74 V\$