Mack gave you a very good answer. The only thing i will add is that this could be just a purchase agreement with an extra long closing date. That is similar but a little different than an option to purchase.

If it doesn't matter to you whether this transaction closes or not, then you might be interested in it. If you seriously want to sell your 4plex, you are tying it up for a year with no guarantee that it will close at all.

If you are interested and want to put the deal together correctly get an attorney to represent you in this transaction. There are a lot of unanswered questions and possible pitfalls in something like this.

Will you be owner financing, thereby giving the purchaser the deed? Will you be doing a contract for deed where you keep the deed? Will you be offering a lease-purchase? Will purhcaser be buying the property subject to the existing mortgage? There are too many unanswered variables in your question. Regardless of how you structure it, you MUST know what you can do legally, therefore you MUST know what the laws of your state are. Some states (specifically, Texas) have all but outlawed lease-purchase and contracts for deed.

Regardless of what the contract calls for, you will still be on the hook for the mortgage should the buyer fail to pay you. If he's paying you, he will surely want the rents to run solely through him. So, in most scenarious he will control the property but you're responsible for the mortgage until he finances his purchase through an institutional lender, which is getting harder and harder to do for investment properties.

You should really seek the advice and counsel of a competent real estate attorney before jumping into this arena. There are far too many pitfalls that can come back to bite you if everything is not done properly.

There's nothing "standard" about the type of deal you guys are up to. Basically, you have to decide what you're willing to give him for his $10K, he has to decide what he wants for his $10K, and you have to work it out from there.

$3200/mo gross income? What's that baby worth, $500K-$600K?
Ten thousand dollars is basically option money. He's going to put another $21K into carrying the mortgage for a year, which would bring us to $31K.

If the tenants stay, you're looking at $38K in gross rent over the next year; if you turned that over to him, he'd be making $7K on the deal even if he just walked away after a year. If you kept the responsibility of maintenance, at least you'd know your property was being taken care of; if you gave him the responsibility, well, who knows?

At this point, you really need to counsel with an accountant to determine if your financial situation warrant being involved in this sort of deal. After that, you can have an attorney draw the deal up.