The acid dissociation constant for acetic acid is 1.8x10^-5 so the pK = -log(1.8x10^-5) = 4.7

How should [OAc-] compare to [HOAc] to make a buffer with a pH of 4.0?

  [OAc^-] >[HOAc]    [OAc^-]=[HOAc]    [OAc^-]<[HOAc]