A 4 letter word where Adding s at the end does not make it plural of the original word but a separate word e.g. ate adding G at the beginning makes it gate which is not anyway related to ate apart from spelling.
So, as per the rules, can we have answers like :: (1) HATE --> HATES, (2) LOVE --> LOVES etc ... ??? They are not plurals .. PS : I don't wanna change my answer. :p
Not easy finding a five letter word ending with S where the other 4 letters are not a noun... Anyway here's one guess. HAVE and HAVES. The latter is used as a single noun meaning people with wealth: "the haves and the have nots". Also, HAVE is not used to mean a single person with wealth, you would say instead "he is one of the haves". 2nd person singular of HAVE is HAS.
One more guess and I'll leave it at that. BRAS and BRASS. That's a better one - brass is completely unrelated to bras - I suppose you could make bras out of brass but I suspect the lady would not be too comfortable. I suspect the 2nd person form of 4 letter verbs is excluded by the example "ate adding G at the beginning makes it gate which is NOT ANYWAY RELATED to ate apart from spelling" (capitalisation added).
Not quite but still open to discussion. Heres in the link is related to here. Quite right but the word I had in mind is Good / Goods