It is strongly advised against modifying the corev4.css file.
This main reasons being that as soon as you apply the next update, your css customisations will be lost as the file will be overwritten, and your changes affect the entire farm.
It's recommended that you create a new CSS file, store it either in a folder in LAYOUTS in the SharePointRoot folder on the server, or in the Style Library in your site collection - and reference this CSS from the AlternateCSS property for the site (through either managed Code or PowerShell), or modify your masterpage to use this new CSS alongside Corev4.