If those who are on PHP 4 have not been hacked yet then it is merely a matter of time.
It is the same as the IE6 movement, up until recently everyone supported IE6 because "people still used it" but the problem was that it caused numerous problems for web developers to support this (everything had to be backward compatible, unclean code, horrid code, coding nightmares) and those computers that did use IE6 were open to numerous day-to-day attacks from the internet that a newer browser would be able to protect them from easily.
And so the drive to stop IE6 (-8 for Facebook, Google, Amazon and many others) started.
It is the same case with API wrappers in PHP, AWS have discontinued their old namespaceless API, even removed it from their GitHub, and they are not the only one to take this move. It is sometimes the responsibility of the API creaters (think of Facebook and Googles and even Amazons, since ecommerce sites are normally the last to enbrace none-backward compatible changes, motto here) to force individuals to not just be happy with an out of date, insecure version of PHP and to seek their lazy web host top actually take precautions to ensure the security and durability of ones own site and data. Through this method we are moving forward in browser technology instead of staying stagnent on IE6.
That is why it is sometimes good to push for a new version irrespective of what needs to be supported.
All PHP functionality is backward compatible as such there should never be problems upgrading PHP, hell I am admining sites written in PHP 4.0 running on PHP 5.5, using PHP 5.5 code where it is possible.