You found that to make a buffer with a pH = 5 you need [OAc^-] > [HOAc].

For acetic acid:

pK = -log(K) = -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]