Did you update the theme when you updated WordPress? A new version of each theme has been made available from your ThemeForest account to correct any problems experienced with V.4.
Make sure you copy and save in notepad or a text editor your custom css so you have a copy of it.
As far as I’m aware there were no changes in the css structure in the update, so your css should work as normal. If it doesn’t it could be you need to be more specific with your css selector. If you can provide a link to your site we’ll see what selector is currently being used and how to override.