If you're using any caching within Joomla, make sure you've cleared that. Our Javascript has changed a lot in the 4.x release, and if you're using a minification or compression script, that could cause the old 3.x Javascript to still load. Additionally, we'd recommend clearing your browser cache, though this shouldn't be necessary.
If that doesn't work, let us know the URL of a page with the problem, and we'll gladly investigate further.
Thanks,
Alex