The Godfather II Hints for PlayStation 3
Although you can only get lvl 3 weapons in the single player campaign you can get lvl 4 weapons but only online by completing lvls there.
To make your business taking infinitely more easier, just take care of the made men first. Sure it takes awhile due to all the favors, but once they are all gone, or the majority of them are, it's easier to just take over business and send your men to places that are being attacked. Then when you attack the compounds, just execute their butts. Its only business :p