if the bookshelf, desk, and general attic area don't seem to have any obvious clues, pay close attention to runi's dialogue -- sometimes their musings are more than just flavor text.
if the 5th symbol isn't one of the remaining ones in your current attempt, then i'd suggest thinking over the other 4 symbols and what made you so sure about their position. there might be something you missed. but we promise the answers are all in this room!