It happened with my JW Marriott Rio de Janerio reservation. I selected 3 people but it kept reverting to 2. When I inquired about this... the hotel only allows a max. of 2 people per room. So even if you select 3 and you get a confirmation via email showing 3, if you go back and check upcoming reservations on your profile you will see the maximum the property allows per room.

You might want to double check with the property if they allow 4 in a room.