There is a lot to be excited about with PHP7 - the performance gains are significant and we are eagerly awaiting mainstream adoption. With that said, IPS4 utilizes several third party libraries to provide functionality across the suite -- one of these libraries is HTMLPurifier. The developers of that library are not yet supporting PHP7 and are specifically recommending PHP5. While we have resolved any known IPS specific issues within the suite for the next release, we too must recommend that you remain with PHP 5.4, 5.5 or PHP 5.6 until which time third party library developers update their libraries with full PHP7 compatibility. We will provide an update as soon as full support for PHP7 is given.
Please note that IP.Board 3.x does not work at all on PHP7. There are no plans to release a PHP7 compatible update to IP.Board 3.x at this time.