Actually, and simply, a Picardy is the ending with a major chord of a tune or section in minor which would normally end in a minor. Picardy's are cadential. Was called a surprise, way back. One can take the idea further, or is it farther, in modern music.
IV I is called a Plagal cadence. (a memorization tool, is Amen)
Keep the Beatles coming! It's a great source to learn anaylization of harmony. Great writers, (Beatles) like many others.