As you can see (click to enlarge), the third and fourth floors are where the four rental units are (three one-bedroom apartments and one studio, presumably the former home of Jim Couri):
The new owner should fold the apartments on the third floor into the living area of the main residence. One apartment could be turned into a nice home office, and another into a small home gym. The fourth floor could be left as apartment-style space, but one could be a guest suite and the other could be for the domestic help.

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Neither apartment should be rented out. If the new owner wants to enjoy a dream home, he should avoid the nightmare experience endured by George Pavia as landlord.
If you can afford to buy a house like this, you can afford to cover the mortgage — if any — without any help from tenants. Paying $20 million for a townhouse means never having to say “your rent is due.”
The Nightmare on East 73rd Street Is Over: Pavia House Sells for $19.5 M [New York Observer]
A House Divided: Uncivil War on E. 73rd [New York Times]
UES townhouse, at center of extended legal battle, hits the market [The Real Deal]
18 East 73rd Street [StreetEasy]
Sotomayor Reports $1.9 Million in Income from Royalties [The BLT: The Blog of Legal Times]
Earlier: Lawyerly Lairs: Justice Sotomayor Está En La Casa
Lawyerly Lairs: A Small-Firm Lawyer’s Big-Ticket Apartment