Two cells are connected in opposition as shown. Cell E 1 is of 8 V emf and 2 Ω internal resistance; the cell…

Two cells are connected in opposition as shown. Cell ${\mathrm{E}}_1$ is of $8 \mathrm{V}$ emf and $2 \Omega$ internal resistance; the cell ${\mathrm{E}}_2$ is of $2 \mathrm{V}$ emf and $4 \Omega$ internal resistance. The terminal potential difference of cell ${\mathrm{E}}_2$ is ______ $\mathrm{V}$.