see here:

Since html.parser is not the same parser as SGMLParser, you may find that Beautiful Soup 4 gives you a different parse tree than Beautiful Soup 3 for the same markup. If you swap out html.parser for lxml or html5lib, you may find that the parse tree changes yet again. If this happens, you’ll need to update your scraping code to deal with the new tree.
Method names

renderContents -> encode_contents
replaceWith -> replace_with
replaceWithChildren -> unwrap
findAll -> find_all
findAllNext -> find_all_next
findAllPrevious -> find_all_previous
findNext -> find_next
findNextSibling -> find_next_sibling
findNextSiblings -> find_next_siblings
findParent -> find_parent
findParents -> find_parents
findPrevious -> find_previous
findPreviousSibling -> find_previous_sibling
findPreviousSiblings -> find_previous_siblings
getText -> get_text