we can use the dilution equation to find the new concentration of the solution
c1v1 = c2v2
where c1 is concentration and v1 is volume of the concentrated solution
c2 is concentration and v2 is volume of the diluted solution prepared
c1 - 4 M
v1 - 2 L of the concentrated solution is taken to be diluted
v2 - final volume of diluted solution - 2 L + 6 L = 8 L
substituting these values
4 M x 2 L = C x 8 L
C = 1 M
new concentration of the diluted solution is 1 M