Wordle is an online game that requires players to guess a five-letter word in six guesses. With the rules of the game still in place (and with six guesses), would the game become easier or more difficult if it was, instead, about four-letter words or six-letter words?

I have no reference for this, but since the difficulty of the game is based on strategic application of probability, I can apply those principles. Shorter words provide less feedback with each guess, and there are fewer possible words from which to choose; however, if you have the right letter in the wrong spot, there aren't as many other positions to check out. As the word becomes longer, it is easier to obtain relevant information on each guess. If there are too many letters, then it becomes more difficult to find the right location for a misplaced letter. This is offset by the fact that there are fewer words from which to choose, so your chances of a blind guess having a number of letters correct is increased. With more than ten letters, the game would quickly become totally routine if your starting word is carefully chosen.

Consider the extreme case of a single letter (not requiring it to be a word) - if you know what single letters the game considers acceptable as words, you then know there is a one in whatever chance of guessing it correctly. If it takes the strict rule that only a and i are word, then you have a one in two chance. However, many dictionaries list each letter of the alphabet as an entry, so that is 26 possibilities, and you have very little chance of getting it right, since there is no strategy, just blind guessing.

With two letters, you have a restricted number of words to consider, but still only get feedback on two letters at a time. If you manage to get one right on the first guess, you still have to make guesses on the second one. At least knowing that the letter is right but in the wrong spot is as useful as knowing that it is rightly placed.

With three letters, the relative frequency of use for each letter starts to come into play, and you can make initial guesses that are likely to give meaningful feedback. I quickly found a list of over 15,000 three-letter words, so there are plenty from which to choose, and you may need several guesses to actually find a letter in your word, before you can start trying to put it in place and consider the others.

The same source gives just under 150,000 four-letter words, around 160,000 five-letter words, and only 86,000 six-letter words. At this word length, you can get a significant number of the frequent letters into your first guess, and if that falls flat your second. This means the number of viable words from which to guess has been nicely reduced. There are still residual guesses that allow you to place the letters you know and consider the possible words to fill in the spaces.

Since there are about the same number of four- and five-letter words in play, the increased feedback in the five-letter game makes it more winnable. The six-letter game has significantly fewer possible words, and good strategy should make it easier to win.

To get the word right on the first guess, four letters would be (relatively) easier than five, and six would be easier again. According to the freedictionary.com these are the number of words:

4 letters - 149,165 words
5 letters - 158,390 words
6 letters - 87,151 words

https://www.thefreedictionary.com/4-letter-words.htm
https://www.thefreedictionary.com/5-letter-words.htm
https://www.thefreedictionary.com/6-letter-words.htm

According to this website, the creator of Wordle has programmed 12,972 five-letter words in to the answer pool, of which 2,315 are accepted as solutions:

In a recent interview with BBC Radio 4, Wardle revealed that he programmed the game to accept more than 12,000 possible guesses (based on the number of five-letter words in the dictionary), but only about 2,500 of those words are included in the game's pool of randomized acceptable solutions. More specifically, the game's source code indicates that there are 12,972 acceptable guesses, but only 2,315 solutions, according to online analysts."If your strategy is just to guess one of the 12,972 words at random, your chances at getting it right are only 1 in 12,972, which is less than a hundredth of a percent," explained Aaron Berger, a Ph.D. student in mathematics at Massachusetts Institute of Technology (MIT), and an occasional Wordle player.

https://www.newsnationnow.com/us-news/what-are-the-chances-of-winning-wordle-on-the-first-guess/