This is open to Premium and Basic members, it is just that Premium members can easily see the statistic that is required. Go to: Your Profile -> Quick View -> Statistics, and then scroll down until you see a Grid labelled "Difficulty and Terrain of Caches Found". (If you are a Basic Member, like myself, all you will see is a summary, called Your Caching Chronology. In which case you will need to 'team up' with a Premium Member, or, post a Note to the cache page before going for the cache, requesting a copy of your Grid.)
You will need to have found at a minimum 49 caches, but they need to completely 'fill' the grid to (and including) the level of Difficulty 4, and Terrain 4, similar to the Grid below. That is, you need to have at least one cache in each of the different levels and combinations of Difficulty and Terrain 1, 1½, 2, 2½, 3, 3½, and 4.
Please add a picture of your Grid to show that you qualify.
I did use G Cookie's foldable TOTT to help place the original cache. This is a similar Mk II cache, but used an extension TOTT this time. Bring a minimum of a Minnesota Mining and Manufacturing TOTT, preferably PLUS TWO.
That's it! Simple! The Difficulty of this cache is probably D1½, but reflects the difficulty in achieving this Challenge. The Terrain, well, you can decide if that is warranted when you bring your TOTT along to get it.
(I was not surprised, as he was one of the few that had been doing the 4B Playground caches that I knew qualified already!)