Hi,
It can be done, but as far as I know, hasn't been done yet. You would need to change 4images to read/write to phpBB's users table, instead of it's own, and edit the sessions support in them both so that the sessions can be carried over from 4images to phpBB.
That's just the basic's of what would need to be done, I'm sure there is more that needs changing as well.
Thanks,
Alan.