How to read the slash in “1/4 satin”?
It would be referred to as "1 in 4" satin and refers to the weave pattern.  4/1 satins would be warp faced twills and 1/4 satins would be weft faced twills.



For more information on this you can read all about it at  https://www.textileschool.com/articles/125/twill-weaves