HI Mansi, Lets see how we can approach for such a problem. The symetry of the problem suggests that the four charges will form a square (let say of side a). For general solution lets take length of string to be l, mass and charge of a signgle charge as m & q.

 

Electric force (F_e) on any given charge because of other three

= kq^2/(2a^2)               -------(from charge digonally opposite)

+2*kq^2*sin(45^o)/(a^2) -------- (from two adjecent charges. sin(45) comes as only component in digonal direction will add up)

 

Gravitational force on mass=(F_g)=mg

Resultant of both is balanced by tension which acts along the string. Let this string makes an angle theta with vertical. Then

sin(theta)=(a/root(2))/l = F_e/root(F_e²+F_g²)

 

Solve t his to get a.

Minimum distance between charges =a

Maximum distance = root(2)*a