When you meet Leonardo, he will offer up one of the Codex Inventions for purchase. Select one of them to purchase. When the sequence ends, access the DNA and replay Sequence 4 Memory 8, "An Unexpected Visitor", again. Once the sequence starts, skip it and you will return to when you were offered the inventions. The first one selected is grayed out, but you will have two options. When sequence is over, do it again to get the last item.