Personally, I wouldn't bother to swatch for a blanket. An inch or two isn't going to make a difference for a dog blanket, but it would, of course for a sweater.
If you really want to, though, check what it says for the gauge directions and cast on maybe 10 more stitches. Then knit for a few inches and measure your stitches.
I don't think I've every knit a full 4 inches for a swatch, since row gauge can only really be adjusted by knitting more/fewer rows. I check out what I'm knitting and decide what to do about it.