Active support for PHP 5.4 ended on September 14, 2014 and active support for PHP 5.5 will end on June 20, 2015. The active support for PHP 5.3 already ended on July 11, 2013. By the time PHPUnit 4.8 will be released, the only actively supported version of PHP will be PHP 5.6.
The next version after PHPUnit 4.8 will not support PHP 5.3, PHP 5.4, and PHP 5.5 anymore. As PHPUnit follows Semantic Versioning the major version number must be incremented when the minimum required version of PHP is increased.