I chose to use the four numeric rings (digits 0 - 9), but an interesting puzzle can be made from the rings having alphabetic characters.
The first ring has the letters A - J; call this type 1 ring. The second and third rings each have the letters K - T; call these type 2 rings. The fourth ring has letters U - Z and four special symbols. We'll ignore the ampersand, hyphen, blank and exclamation mark, but you might wish to explore such things as L&ER, for "lander" on your own.
Although foolish from a security standpoint, the use of a real word has mnemonic value. The foolishness is especially true as the specific hard-and-fast choice of the sequence of types, which can't be altered by someone merely opening the lock, limits the possible words that might be the key. For example if the sequence of types fixed onto the rings was 3221, few common words would be possible, like ULNA, VOLE, VOTE, WOKE, WOLF, WOMB, WOOD, WOOF, WORD, WORE, YOKE, YORE and ZONE. A potential thief could go through these quickly.
So categorize, by the sequence of type of ring, the possible 4-letter words that might be chosen as the unlocking key, given that there are one each of types 1 and 3, and two of type 2.
There are no tricks like treating W and M interchangeably, as, first of all, one is not really the upside-down of the other, and second, the one being used would be given away by the fact that all the other letters would be upside-down also.