....
Many, if not most, of the cruising sailboats I see offshore have some
sort of visible radar reflector. They typically show up very well at
4+ miles. I believe we'd see most of them without a reflector based
on my experience picking up small power boats.
....

Sea conditions play a huge role here. If there are small power boats
about you aren't at sea, you're near coastal. Over the last couple of
decades of voyaging and delivery work and through many bridge to
bridge communications I'd say that there is about a 50/50 chance that
a large ship that is looking for you will see a mid-sized cruising
boat with a radar reflector at four miles in anything less than ideal
conditions in the open ocean. It is extremely rare in my experience
for a ship to spot a small vessel offshore before being hailed. My
experience has largely been in the Pacific and in many cases in remote
parts of the Pacific and may not apply to the East Coast, but I think
it is worth keeping in mind that as the seas get large and the radar
less effective and the watches get less expectant of seeing vessels
and less diligent your chances of both seeing and being seen go way
down over what you might be used to in protected waters.

-- Tom.