Yeah Calvin, how'd ya know?
The fourth-finger in thumb position and third finger in normal position weren't really things that they had a definate time to use. They strongly enforced that it all depended on the circumstances, which for them tended to be 'once in a while' to 'often'. The whole idea was not to limit the tools they had "at hand" if you will.
Very interesting concepts, but I'd like to become comfortable all over the bass before I start throwing a fourth finger into thumb position, or a third into the easier areas.
As for the petrarchi... I don't know. Which study # is that in the book?