Basically, the way a minimum version is located is to lookup the various commands used via php.net, each command listed there has a minimum supported version listed; you would check each command and whichever shows the latest lifecycle (here 4.3) would be the required version.
Where it may work on older versions of php, some features will not work properly as php woudln't know what to do with the coding, and thus the feature of the forum you're trying to use woudln't work.
Note however that in most cases it woudln't "crash your forum", the feature you're trying to use just woudln't work (or at least woudln't work properly).
Also keep in mind that PHP's current version is nearing version6, if your isp is using something less than 4.3 something is way wrong, not to mention there was a fixed security isssue in 4.3 due to some bad coding on phpbb that was all the rage last year.
