Catalyzes the third step in the biosynthesis of pseudaminic acid, a sialic-acid-like sugar that is used to modify flagellin. Mediates N-4 acetylation of UDP-4-amino-4,6-dideoxy-beta-L-AltNAc to form UDP-2,4-diacetamido-2,4,6-trideoxy-beta-L-altropyranose (By similarity).