I read in a book: 四声(sìshēng) which is translated in the book as "the four tones" Somewhere else in the same book, I read: 三声变调(sānshēngbiàndiào) which is translated as "The third tone sandhi"
In the first example, (number+noun) is translated as a cardinal number. But in the second example, (number+noun) is translated as an ordinal number. It's just confusing. How should one say "The fourth tone"? and make it different from "the four tones"?
The word 四声 means all the four tones of classical or modern Chinese.
普通话的声调有阴平、阳平、上声、去声四类，叫作四声。
At the same time, 一声 (that is 第一声 or 阴平), 二声 (that is 第二声 or 阳平), 三声 (that is 第三声 or 上声), and 四声 (that is 第四声 or 去声) are sometimes used to indicate single tones.
The only ambiguity can be related to the use of "四声" instead of 第四声 or 去声. I understand that this can be confusing, but I have never found any situation in which the meaning can be reasonably misunderstood.
Yes, confusion is possible. The long forms 第X声 "the Xth tone" and X个声调 "X tones" are unambiguous, but X声 can mean either "X tones" or "the Xth tone".
In Chinese linguistics, when discussing 方言 topolects, X声 usually means "X tones" as there is a standard set of terms to refer unambiguously to each individual tone (阴平 etc). You can number the tones but nobody does it as there are multiple conventions which means people just get confused. You more commonly see numbering in romanization schemes, but these all have their own well-understood definitions, are only used for speakers of that topolect anyway, and (in my experience) only as tone marker substitutes in the romanizations and never read as numbers.
These don't apply to the average Standard Mandarin speaker. To them there's just four tones and no possibility of anything else. So, people really only need to talk about which of the four tones it is. And technical terminology is, well, technical, so people are just taught to use the simple system of numbering tones. To ordinary people, X声 means "tone X" unless the context indicates otherwise (though I think laypeople are much more likely to say X个声调 for "X tones").
