[Borek]

No idea what you mean.

[Saint]

I didn't finished my reply my internet was cut.
Now...
That typo you gave to me the Hederson one exists only when the approximations work.
Here after ionizations and with C H₂SO₄'s concentration we have:
1) C+x=10^-4 (from the ph)
2) 10^-2=x*10^-4/C-x  (from the second ionization)
Well that Hederson typo is the 2) but with the approximations:
3) 10^-2=x*10^-4/C

If we solve 1-3 we find that C=x which is impossible
If we solve the 1-2 we take "reasonable" results which I'll write down tomorrow...
I hope you understand... Goodnight...

[AWK]

H2SO4 with pH 4 and NaOH with pH 10 practically forms Na2SO4 with concentration 2.5 10-5 M (error less then 0.5 %). I think this problem can be approximately treated as hydrolysis of Na2SO4 instead of buffer Na2SO4/NaHSO4 far outside of reasonable bufferring capacity range.

I think Borek can check both possibilities with his program