Knit a larger size, though you'll need to do the lengths to the measurement of the size you'd normally do. You might try your yarn on size 5 or 6 needles and may get closer to the right gauge. The gauge is 18 sts per 4" or 4½ sts per inch. Cast on 24 sts and work for several inches. The aim of a swatch is not to make a 4" square, but to see how many sts in 4". You can use a measurement for the length rather than rows (convert the rows to inches) so the row gauge isn't quite as crucial.
sue- knitting heretic