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We compute the fermionic contributions to the cusp anomalous dimension in QCDat four loops as an expansion for small cusp angle. As a byproduct we alsoobtain the respective terms of the four-loop HQET wave function anomalousdimension. Our new results at small angles provide stringent tests of a recentconjecture for the exact angle dependence of the matter terms in the four-loopcusp anomalous dimension. We find that the conjecture does not hold for two ofthe seven fermionic color structures, but passes all tests for the remainingterms. This provides strong support for the validity of the correspondingconjectured expressions with full angle dependence. Taking the limit of largeMinkowskian angle, we extract novel analytic results for certain terms of thelight-like cusp anomalous dimension. They agree with the known numericalresults. Finally, we study the anti-parallel lines limit of the cusp anomalousdimension. In a conformal theory, the latter is proportional to the staticquark-antiquark potential. We use the new four-loop results to determine partsof the conformal anomaly term.

Matter dependence of the four-loop QCD cusp anomalous dimension: from small angles to all angles