The reference is to the pages of the [Venice printing of the Talmud Yerushalmi](http://hebrewbooks.org/45298) (5283/1523). Each folio is divided into four columns, two on each side of the page, and these are the numbers of the amud (1 and 2 on the front, 3 and 4 on the back).

The sentence quoted from page 53d (Sukka, chapter 3, law 6) appears in the middle of the left column of [this page](http://hebrewbooks.org/pdfpager.aspx?req=45298&st=&pgnum=235).