I was curious about the number of clubs in North America that have four courts or more. Here are the ones I can recall off the top of my head. Please feel free to add to the list.
Buffalo Croquet Club - New York
Merion Cricket Club - Pennsylvania
Mission Hills Croquet Club - California
National Croquet Center - Florida
Ponte Vedra - Florida
Sarasota Croquet Club - Florida
Tulsa Croquet Club - Oklahoma
Think I just found another one --
Chesapeake Bay Croquet Club -- Virginia
More info on the Chesapeake Bay Club --
I see that Oxford lists PGA as five courts.
In North Carolina, both Pinehurst and Bald Head Island have four lawns. In each case the fourth lawn is not ordinarily set up for croquet, but is at the same location and has a good playing surface.
I think there is still a four-lawn facility in Victoria, BC.
Various clubs with grass tennis courts that can at least occasionally be used for croquet include Piping Rock and the Meadow Club on Long Island, and Sherwood in Thousand Oaks, CA. The latter two have been used for national championships before, but arranging that takes effort and insider connections. Oh, and of course the Tennis Hall of Fame in Newport, once a major venue for the USCA.