would the ph shift observed with the buffer hace been greater less or the same if 0.4 M buffer had been used instead of 0.2

It will remain the same.

Explanation:

A buffer solution is a kind of solution that has resistance for changes in the pH of a solution. It is a solution of weak acid and conjugate base or it may be otherwise.

H^+ + A^- <> HA.

Buffer solution aids in stabilizing pH and an example of a buffer solution is the human blood.

The higher the concentration, the lower the ka and vice versa. Thus, when 0.4M is being used instead of 0.2M the pH will be the same because when dealing with buffer solution, it is all about the value of (salt) / (acid) rather than on the molarity of one specie alone.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.