Projecting a colour original onto ortho should only require the same filtration as the original scene done on ortho. Green, possibly. The enlarger lamp colour temperature may have some influence, but tests will determine that.
I assume that the 5x4 is the final size - for a contact print? Emulsion to emulsion contacting is the usual approach, and then enlarge, but I can see a few processes where going straight to a final larger negative might appeal.