masih1994 I think it's not that simple. PHP 5.4 is not even in active support. Only security fixes will eventually come out.
Sources : http://php.net/supported-versions.php
Flarum is a "nextgen" forum, so it sould use a "nextgen" (or more "currentgen") language.
I find that really weird that a webHost still propose PHP version under 5.5. (my opinion)
But i know that not all scripts are compatible to new versions of PHP.
Choose your host carefully, not the cheaper. 🙂
BTW i'm still waiting for PHP 7 ! http://php.net/manual/en/migration70.php