When CH3COOAg(s) is dissolved in water an equilibrium is established with the solid salt at a given temperature forming ions as follows.

CH₃COOAg(s) ⇋ CH₃COO^- + Ag⁺ ...[1]

If the salt is dissolved in buffer solution of pH=4 ie [H+]=10^-4 then acetate ions in the solution goes to combine with H+ ions as follows and proceeds towards another equilibrium state

CH₃COO^- + H⁺ ⇋ CH₃COOH.....[2]

Both the reactions establish a overall equilibrium state.

Let at the final equilibrium state the concentration of [Ag⁺] be x(M). This will  represent the solubility of the salt in buffer solution. If a(M) be the concentration of acetic formed at equilibrium then concentration of acetate ion will be (x-a) (M)

Concentration of H⁺ ion remains unaltered due to buffer action.

So for equation(1)

K_sp=(x-a)*x.

=>(x-a)*x=10^-12 ......(3)

And for equation (2)

1/k_a=a/((x-a)*10^-4)

=>3/10^-4=a/((x-a)×10^-4)

=>3=a/(x-a)

=>3x-3a=a

=>a=(3/4)x...(4)

Combining (3) and (4) we get

(x-3x/4)*x=10^-12

=>x²=4×10^-12

=>x=2×10^-6 M