In perturbation theory, masses need to be renormalized and therefore depend on the chosen scheme.
It has been shown that the perturbative expansion of different observables shows a better perturbative expansion,
when specific renormalizion schemes are used.
After giving a short overview of commonly used renormalization schemes, we report on the novel calculation of the on-shell mass renormalization constant up to four-loop order allowing for a second non-zero quark mass.
For a subset of color factors we obtain analytic results in terms of iterated integrals over rational and square-root valued letters.
The full renormalization constant is obtained semi-analytically via precise overlapping series expansions for arbitrary quark mass ratio.
In the limit of the same mass we recover and improve results which have been obtained previously using numerical methods based on sector-decomposition and Mellin-Barnes integrals.