Force between two charges is given by
[tex]F =\frac{kq_1q_2}{r^2}[/tex]
[tex]F =\frac{9*10^9* 1* 10^{-6}* 1 * 10^{-6}}{4^2}[/tex]
[tex]F = 5.625 * 10^{-4} N[/tex]
Now in order to find the acceleration of each mass
we can use
F = ma
[tex]5.625 * 10^{-4} = 1 * a[/tex]
[tex]a= 5.625 * 10^{-4} m/s^2[/tex]