I orriginally read it is a double decrease (sl1, K2tog, PSSO) which indeed would be a decrease of 4. If their is no glossary with the pattern explaining the term i swould contact the publisher or designer for a clarification. Either the end stitch count is incorrect (and it should be 84) or the PS meens something different, such as simply purl stitch, which you need to find out about.
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