A list of hotels in each Marriott category can be found on-line under Marriott Rewards. You might find some ideas there, and then could ask about specific properties.

Sometimes it is hard to offer assistance on a general question across all brands and continents. .

And although it might be hard to find a nice resort at a Cat 4, there are certainly some decent properties in beach towns that are Cat 4's. And let's get real....free nights without use of points can make decent look pretty good.