I think I have this right, but I just want to confirm.
Say I invented two of the ladder-type inventions (Bronze and Silver). Then the Copper one comes up. I can start working on that invention and don't have to put any apprentices in that lab and it'll get invented automatically because of the 4 week discount.
Almost. I invented the Bronze and Silver ladder type inventions and the Copper. I was busy working on a different Gold invention and another Bronze when it showed up. I could have invented it in one turn, but never got the chance.