Whenever I get my free night, I incorporate them for the best value in my planned travel, and don't necessarily plan travel around them. Going out of your way to use a Cat1-4 will cost more than any extra value gain.
There are quite a few decently expensive Cat 4s, but it seems a bit too often for coincidence that when they do get expensive, the don't allow award bookings anyways in my experience.

(I give the same opinion about the travel bloggers that raved last summer about the 2-free cat1-7 as such a great deal with the cc signup, and better than the 40,000 points, because you could use them at PH Maldives -- That's great if you are already planning on forking thousands in travel costs and 1-2 days towards making your way to the other side of the globe, but for most people, the 40,000 points offerred a more realistic better redemption)