From https://phpunit.de/

PHPUnit 4.8 is no longer supported.
It became stable on August 7, 2015.
Its support ended on February 3, 2017.

You can find out what was new in PHPUnit 4.8 in the release announcement.

PHPUnit 4.8 was supported on PHP 5.3, PHP 5.4, PHP 5.5, and PHP 5.6.

MW core has

"phpunit/phpunit": "4.8.31",

4.8.34 seems to be the last available version... Probably worth updating in core and in CI stuff

I know we're still using 4.8 as we need PHP 5.3 support... Roll on May 2017 when we don't need to support REL1_23 :), and can use the new old stable of 5.7 ;)