I was thinking of adding a note to the panel puzzle that says there are four pieces, maybe that would get people looking without giving it away?
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To be honest, I find the "look in every nook and cranny until you find the one thing you're missing" style of puzzle to be really unfun. It would have helped to know that there were four pieces, yes, but even after I knew there had to be a fourth, it took me several minutes to find it. If you were willing to put even more work into this game than you already must have, then my recommendation would be to lock it behind another alchemy or mathematics puzzle. Those were really neat! But if that's too much, than all I can really think of would be to put it in a less concealed spot. Then again, nobody else seems to have had trouble with it, so perhaps chalk that up to a difference in sensibility. I reiterate, though, I loved the game on the whole.