Check out
The ABC's of Real Estate Investing. I can't remember the author, but it's a Rich Dad Advisor book, so it has that purple-and-gold cover.
He does assume you're doing commercial property, but the same rules will apply to a four-plex; you'll just have to do things in slightly different orders and use different terminology. He offers step-by-step checklists, tells you what data you need, and runs through how to calculate profitability and offer price from the data.
Also, check with local investors on how much trouble the extra regulations are for commercial. In Colorado, at least, the need for paperwork is
easily outweighed by the fact that you're not personally responsible for paying for commercial property. To buy a four-plex, you have to show the bank enough income to pay the mortgage on a million-dollar property. To buy a five-plex, you have to show the bank that the property will pay for itself. Even though your intention with a four-plex or even a single family home is the same, the bank won't give you the same loan.
But that's in Colorado and (at risk of severe stereotyping) I admit that the protective, liberal, people-are-too-dumb-to-think-for-themselves attitude in Massachusetts may have made commercial property too difficult for investors.
(Note to liberals, democrats, and people in Massachusetts: please don't flame me! I voted democrat!)
My turn: Where do you find listings for four-unit apartment buildings?