Some suggest (and have) a "quint" valve for a 4th valve...and I'd guess that those people use 1-3 and 1-2-3 (with epic pulls) for those two 1-3 and two 1-2-3 pitches.
Does anyone mess around with a regular "quart" 4th valve (yet too long?) whereas it might over-correct for 2-4 only slightly under-correct for 1-4, and (for lower pitches than those) "fishing around" (as with other non-comp systems) to find the best compromises?
...I remember that the Willson (non-comp.) 2704 (four-valve non-comp in-line euphonium) featured both a 4th circuit that was too long, as well as a 2nd circuit that was too long...ick.
I'm not suggesting ANY particular (only four valves) workaround system, but am curious to read how some people (who really strive to play all of these involved pitches in tune) manage to get by.