1. The problem statement, all variables and given/known data
500 ml buffer solution with pH 4,0
I want to create the above.

What i have is:
0,10 Molar acid-solution HA with a pKa = 4,35.
0,20 molar of the acids sodiumsalt NaA


2. Relevant equations
pH=pKa-log(c(HA)/c(A^-)


3. The attempt at a solution

pH=pKa-log(c(HA)/c(A^-)

4=4,35-log(c(HA)/c(A^-)

0,35=log x

10^0,35 = (c(HA)/c(A^-)

Wich results in
0,1*x = (10^0,35)*(0,2(0,5-x)

your approach is correct
now take this ratio as the no of moles of HA/Na+A-
molarity is given to you so take out the ratio in which volumes are mixed to give the buffer