N-alpha-acetyltransferase that specifically mediates the acetylation of the N-terminal residues of histones H4 and H2A. In contrast to other N-alpha-acetyltransferase, has a very specific selectivity for histones H4 and H2A N-terminus and specifically recognizes the 'Ser-Gly-Arg-Gly sequence'. acetyl-CoA + N-terminal L-seryl-[histone H4] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-seryl-[histone H4] acetyl-CoA + N-terminal L-seryl-[histone H2A] = CoA + H(+) + N-terminal N(alpha)-acetyl-L-seryl-[histone H2A] Belongs to the acetyltransferase family. NAA40 subfamily.