The four-loop cusp anomalous dimension from the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:mi mathvariant="script">N</mml:mi><mml:mo linebreak="goodbreak" linebreakstyle="after">=</mml:mo><mml:mn>4</mml:mn></mml:math> Sudakov form factor
Physics letters. B