I am local to you, but I am one of those that @Account Closed mentions that does tend to work with larger investors so you wouldn't be a good fit for us - so I'm not self promoting. But if you want to reach out there is the names of a couple local smaller firms that I tend to refer business to (there are absolutely no kickbacks to me - I just want to help people out) that aren't a good fit for us. That said, there is almost no one in NH in the CPA/EA world at a small firm that I am aware of that focuses most of their time in real estate - perhaps very competent in the issues, but not anywhere near the concentration in the real estate world that you'll find from the professionals you could reach out to on this forum - you really can't go wrong.
It doesn't hurt to find someone with local experience in your case if you are able to find a firm that is a good fit and vice versa - the NH Business Tax can be messy - but just about anyone on this forum can help you deal with the other tax issues, and are smart enough to figure out the NH issues with some research or by phoning a friend.
You do have a unique situation where the property itself has been rented already (the 4 unit as a whole), so generally you don't get to track and place the single unit into service on it's own. Additionally you don't get ST rental treatment on that single unit alone - instead one needs to consider the treatment for the entire building.
In your case doing a cost seg study would go back to the day the 4 family was placed in service - whenever you started renting the other 3 units. Any bonus depreciation catch up through the Form 3115 would include an exclusion over all these years for the personal use piece of the building (your unit). You wouldn't get any benefit for this 4th unit now being a ST Rental.
In short, doing a cost seg study on the building may accelerate some depreciation, not end up with a ST rental benefit, and also end up with significantly more disallowed depreciation on the personal rental unit. Perhaps it is going to be worth it, but you won't be able to truly benefit from the short term rental "loophole" in your use case.
Side note - I could swear we've met before - do you attend NHREIA meetings? I'll be headed there tomorrow (5/8) evening.