I think one more pair would be fine; but of course it all depends on your tank's biological filtration.
Do make sure that you either get the seahorses from the same source (if you got the originals from a reputable aquaculture facility), or do a lengthy QT with a gradual introduction and be aware that mixing two different sources of seahorses can cause disease and death in one or both groups of seahorses, even if they are the same species.