If the marking is in a title and therefore is in contracted braille, you should use the letter sign. The only guidance I find in EBAE is "Rule VII. 29. Exception." The second paragraph refers to ordinal numbers with foreign endings, and is, of course, about contracted braille.
If it is in a musical heading or instruction and therefore uncontracted, I don't think you need the letter sign since "r" cannot be misread as a numeral. If it were uncontracted and the first letter after the numeral were one that could be a numeral I'd use the letter sign.
I don't see any reference to using dot 4.