Nothing in this world is impossible, just varying degrees of difficulty. The first thing I would do is to go to the City planning department and speak to someone in building and safety. They likely will indicate that you can do it, however you will need to have an engineer and possibly an architect draw it up to ensure that it will be safe.
An equally important question to ask yourself is if it is financially wise to down size. If this is partially an investment for you, I assume this because you are renting the other three units, then how will this change affect the value when you go to sell the property. The normal assumption is that it will reduce the potential earning potential of the property, thereby reducing its resale.
You may want to hire a real estate agent or appraiser to help you determine the potential affect on value, if this is a concern for you. If it is not a concern, then head to Building and Safety and start getting permits and plans.
You should also be aware that this change may affect the loan that you currently have ton the property, if any, because the lender may see this as a substantial alteration to the improvements that may affect potential resale and therefore their security. You donâ€™t want the bank to surprise you half way through construction by calling the loan and asking for full repayment.