Is the old version of your board still active? If yes post the url so that we can at least have an idea of the size of the board.
Also, surely you know the phpBB version of your board? You have the board since 2009: which version did you start with, Did you install it, did you update it?
Can you post a printscreen of your folder structure (with the size of the folders), also post a printscreen of the contents of the /store folder. If there is an SQL file in there or a backup_ xxxxxx.gz, tell us the size.
A 4 GB database would be exceptional for a board as you described.
Brexit and Trump are old news and currently I have no inspiration for a new signature.
Ow, wait: 3.3 has been released.