The use of fancy hi-tech imaging techniques has revealed a previously-unseen pre-Conquest Mexican manuscript beneath the C16th Codex Selden:
https://www.theguardian.com/technology/2016/aug/21/hidden-codex-reveals-secrets-of-life-in-mexico-before-spanish-conquest
I wonder if the Bodleian Library are thinking about trying the same scanning technique on the other four.