Go back to the bars that now block your exit and speak to Tolfdir who will come to help. Once retrieved, you’ll be trapped inside the room. The fourth artifact is the Saarthal Amulet. Unlike some other followers, Lydia is not essential (immortal), and can be killed without you knowing about it. You might be interested: FAQ: How To Get Rid Of Amulet Of Talos? Does Lydia die in Skyrim?