Hello,
We just performed a Confluence migration on our test environment from v3.4.9 to v4.1.9 and all went well. All plugins were updated and pages display fine. However in page Edit mode the layout is all messed up: imagine a 4 cell table where the header is in the top left cell and the content in the bottom right, the other two cells are empty.
We did not customize the default theme to avoid any issues between migration in order to upgrade frequently.
We need to migrate to v4.1.9 and not later as some used plugins are not yet compatible with later versions of Confluence.