I'd look at the gauge for the pattern to see how many stitches in 4" to make gauge. Then I'd add some to it, at least about 8, knit away for a while and see how it looks. If it's obviously less than 4" wide, I'd go up a needle size or 2.
When you change needle sizes in a gauge swatch a row of garter stitch makes it easy to see where you changed. Do something to mark the needle size you use in any given section of your swatch, I've even written the needle size on a scrap of paper and safety pinned it to the appropriate section.
You'll probably get much better answers, but that's what I do.
What'cha swatchin' for?