In my opinion, the problem is ambiguous, and the calculations are also unfinished. You should calculate solution volumes, not concentrations. It is recommended that the results should be rounded off at the last stage of the calculation. If you round the intermediate results, it should be done fairly safely for calculations: in intermediate results, I would advise leaving at least two significant numbers more.

If we assume that the input data are inaccurate, then we have one significant digit in concentration and in input pH 4.0, pKa is certainly the experimental data and has two significant digits (why?), and the volume of the solution has 3 significant digits if we write it as 100. . The accuracy of calculations with one significant digit does not make sense, so we assume that all input data except pKa are accurate. The calculated volumes, in our case, will have two significant digits (after the final rounding).