4 single engine prop planes with no ADSB?

[AlinaCairns]

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.

[Florida Metal]

turbo prop or piston?

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

[Timothy Aanerud]

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