4 single engine prop planes with no ADSB?

Two days ago I watched 4 prop planes flying in very close 2x2 formation fly directly over me(NYC area).

I immediately pulled up my PiAware, but they weren't reporting anything. Is that normal? What might have they been?

Typically the only thing I don't pick up is military, but they looked more commercial.

turbo prop or piston?

Often formation flights will just show up as one aircraft on adsb exchange and flightradar24

Does your PiAware have both 1090 and 978 tuners? In the U.S. aircraft can use either frequency below 18,000 feet.