Notation in atomic structure
Copied table below from "Quantum chemistry", Donald A. McQuarrie, University Science Books,  2008, page 472

This illustrates the notion that $p^2$ and $p^4$ are equivalent.  



I think the 2,4 superscript in Cowan's book for $^{2,4}S^\circ P^\circ D^\circ$ is a peculiar notation of his. Most other books seem to presume the reader knows the equivalency.