Hi,
Was just wondering why the abstract nouns in both sentences below are countable?
I’ve just had a good sleep. We say this informally, but I wouldn't say it in formal speech or writing. Perhaps we are thinking 'a good night's sleep' or 'a good period of sleep' and we just shorten it. As you say, with three or four, it starts to sound a bit odd.
The word has a different spelling. This is so commonly used that it just seems to me that 'spelling' can be counted in this way. It sounds fine to say 'This word has two different spellings'. Finally, I just looked in my dictionary, which confirms by its definition that it can be counted.
Best wishes, Clive