In contrast to the first three characters, you'll never see 囗 in the wild. Instead, it's an important character component, and there are quite a few characters that build on it. You'll not have to learn its pronunciation, lest you need to write the components of some characters sometimes. If you see a large wooden box in the mnemonic's story of another character, you know it refers to 囗.
囗 is pronounced "wei2", which translates to "wu" + "ei2" in the Gorodish system, so there's Willy Walrus for "wu" and the Eiffel tower for "ei". Because it's the second tone, the setting is just inside the entrance of the Eiffel tower.