Can you link to the pattern or give us its name? A 4 ply yarn may simply mean it's got 4 strands in it and could be the same as the one you linked to. Or it may also mean a 4ply fingering weight which is half as thick.
Even if the pattern doesn't say how many sts per inch the gauge is, you can divide your sts into the number of stitches to see how large around that would make it. If it's larger than the head you need less stitches.
sue- knitting heretic