Homonyms are words that are spelled differently and pronounced the same. There are thousands in the English language, including tons of triple homonyms like "two," "too," and "to," but very few quadruple homonyms. I haven't seen these listed anywhere else, but I have a few:
- ai, aye, eye, I- An ai is a type of sloth. I actually knew that from Scrabble, but didn't realize the pronunciation.
- air, err, heir, ere- This one depends a lot on pronunciation.
- axle, axel, axal, axil- I wasn't even sure this was a homonym at all, but there it is. Besides the wheel type, there's the figure skating jump, an obscure spelling for "axial" and an obscure botany term.
- carat, caret, carrot, karat- Kind of cheap because "carat" and "karat" mean similar things (measurements of precious stones).
- cees, seas, sees, seize- Also pretty good, "cees" is a bit obscure, but you could say "The word 'broccoli' has two cees in it."
- cere, sear, seer, sere- I don't really like this because I think "seer" is pronounced a little bit differently.
- cents, cense, scents, sense, cense "cense" is a bit obscure.
- faes, fays, faze, phase- I'd say "fays" (fairies) and "faes" (rocket engines) are both pretty obscure.
- gnu, knew, new, nu, knew- Also pretty good, although some people pronounce the 'g' in "gnu," particularly free software advocates.
- lase, lays, laze, leis- "Lase" is actually a word meaning "to give off coherent light."
- lochs, locks, lox, lakhs- Iffy because "lochs" and "lakhs" are both kind of foreign/archaic, but definitely all in the dictionary.
- medal, meddle, metal, mettle- Probably the best example, doesn't need any pluralization and these are all reasonably common words.
- right, rite, wright, write- Good except "wright" which is not too common, although derivatives like "shipwright" make sense.
- rhos, roes, rose, rows- Greek letters are pretty useful for coming up with these. "Roe" is a weird one since it is already pluralized, but "roes" is still a word in the Scrabble dicitonary.
- sais, sighs, size, psis- I like this too, although the weaponry "sais" as yielded by Rafael, is not in some dictionaries.
- teas, tease, tees, tis- The last one is a bit unusual, it's the plural of the note "ti" of "do, re, mi" fame, but it's in the dictionary alright.