I made the query at my webhotel and they, as you predicted did not have that option on our webhotel.
I suppose I will have to look for a new webhotel.
Pity but there it is...
oh and Merry Xmas!
If you have the option of PHP 5 you can try going with that. Such a change introduces risk. Hopefully you phpBB installation will continue to work transparently. You may not be able to revert.
It really depends on what libraries are integrated into PHP 5 by your web host. You can simply file a technical support ticket and ask, "If I switch to PHP 5, will the mcrypt library be available?" If yes that would be a compelling reason to upgrade.