It is not a "guild gun" as it is marked as made by L. Westphal in Peine, a town in Lower Saxony in Germany.
The design is quite clever in using a movable hammer nose to fire either nipple. The forward nipple channel would lead to the top barrel, the rear one to the bottom barrel. The spring on the side of the hammer would keep the hammer nose in whatever position it was set until the user moved it to fire the other barrel. I immagine one would fire the top two barrels, then cock the hammers and move the noses of both hammers back to fire the lower pair of barrels.
The gun was obviously a high ticket item; the engraving is good and the Damascus barrels are beautiful in appearance (if not necessarily in strength today).
A very nice gun and I will guess a value of at least $2000, perhaps more. As noted, such items are hard to sell because American collectors are rarely interested in them.
Jim