N644 is taken. It's currently assigned to a 1982 Cessna R182 registered in Livermore, CA.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N644 has been worn before. Past registrations from FAA deregistration records:
Tails one nudge away from N644 that aren't assigned or reserved: