I bought a condemned 4 unit apartment building in New Hampshire. I have refurbished and rented 3 units...?

This has taken twice as long and cost 3 times as much as I had budgeted. Now I've invested all of my savings. After many visits from city inspectors I've finally rented 3 of the 4 units.
I have to concentrate my "real" job for a while. So, I want to wait till the spring to re-do the last unit. I've shut off the electricity and water to the 4th unit. There is still some water in the pipes under the sink and comode. Do I have to keep the place heated? If so to what temperature? What else should I do to protect my investment while rebuilding my savings account?

If you have turned off the water supplies to that unit, leave the faucets open and don't be too concerned. Regarding the drains holding residual water, simply get a couple gallons of inexpensive RV antifreeze and pour it down the sinks and into the toilet. The stuff is not poisonous and will prevent freezing unless it gets bitterly cold (-10F or lower).

If no one is inhabiting the 4th unit why do you have to keep the heat on at all? I understand that there might be a possibility of pipes freezing but if you've removed all the water you won't run the risk of the pipes bursting. However, if the unit is a "construction site" and you have paint cans, paint thinner, and other flammable objects inside, I'd remove them to prevent a fire, make sure the doors have good dead bolts on them to prevent vandals and don't keep anything of value inside the unit. Visit it often to make sure you don't get squatters or vagrants moving in. Other than that I don't think you have to do much else.

My friend owns a cabin in the mountains of NC, and only visits a couple of times a year, however, when ever he leaves it he makes sure that the pipes are empty during the winter so his pipes don't burst, but he doesn't put the heat on at all, he shuts it off.

Offer the 4rth unit as a rent, and let the renter pay the renovation- deductible on the monthly cost. Example if it is 2,000 USD monthly, let him rent it for 1,000 USD Monthly- 1,000 deductible on the renovation as long as it is being paid.

In this case, you have a caretaker that pays you monthly!