$\frac{{{E}'}}{E}=\frac{\left| {{q}'} \right|}{\left| q \right|}{{\left( \frac{r}{{{r}'}} \right)}^{2}}\Rightarrow \frac{E+6}{E}={{\left( \frac{4}{2} \right)}^{2}}\to E+6=4E\to 3E=6\to E=2$