Most patterns use a measurement for lengths, so the row gauge isn't as important as matching stitch gauge.
A 4ply yarn is generally considered to be a fingering or sock weight; the #4 or 4ply worsted yarns are considerably heavier, like more than double the thickness. So maybe you're a little confused on that. Here's a page
that might help.
As to gauge in general, CO 4-8 stitches more than the number of sts given for 4" and don't include the edges when you measure how many sts are in 4". If there's too many, the stitches are too small and you need a larger needle; if there's not enough, the sts are too large and you need a smaller needle.