I just came across the following sentence;
How many grade four kanji have you memorised?
Is that formal? Should there better be a hyphen like 'grade-four'?
The sentence presumably refers to a standard Japanese educators have adopted regarding which kanji are to be taught at which elementary school level.
Each country has its own educational system and terminology. What is 4 年生 is more or less equivalent to what might be called fourth grade in the United States, grade four in Canada, year five in England, and so on, but so far as I can tell, there is no single standard way of translating Japanese grade levels into English.
I might slightly prefer the American usage because the Japanese education system was reformed along U.S. lines during the postwar occupation of Japan (and both were originally organized along Prussian lines). That is a quirk of my idiolect, however, and not a real-world convention.
When a compound is used as a modifier, it is conventional to hyphenate the words to indicate more clearly that they communicate a single idea. For example, a
refers to a drink made through the process of top fermentation, whereas a
top fermented drink
might be that, but might also be interpreted as some unspecified type of fermented drink which is of top quality or popularity.
I would wager there is a much stronger association of grade with a number than with the word kanji in any English-speaking country, and so the hyphen may not be needed to disambiguate the meaning of fourth grade kanji. It is unlikely to be confused as meaning fourth grade-kanji. As such, I would rate whether or not to include the hyphen as a matter of style, with either the open or the hyphenated form acceptable provided you are consistent in its use.