The level styles used in OmniOutliner 3 have been added back into OmniOutliner 4. Based on user feedback and our own exploration, we decided it was best to restore these. We have not removed any functionality from named styles themselves though and existing files will continue to function the same way. However, your existing files will likely have two sets of "level styles" now. The named styles labeled Level 1-5, and "Level X rows" right underneath the Whole Document style.
If you were using the named styles I would recommend switching to using the real level styles instead. Unfortunately, there is not an easy way to make this transition at the moment. As always, if you need any help with this, feel free to contact support
Thank you for sending in your feedback!