Guage question dealing with tension

I have a canadian knitting pattern for a baby sweater and on the tension part, it says 24 stitches and 30 rows should equal 4 inch on a 4mm needle. Do I have to go and do all 30 rows, to find out my tension…meaning do I need a larger needle or smaller needle to find out my tension? If I get too many stitches per inch, should I go down a needle gauge or go up?

For a sweater, I would definitely do the swatch.

I believe that if you have more than the given number of stitches/rows to make the 4 inches, you would go up in needle size. (Now watch me be wrong…sometimes this stuff gets me going backwards, lol!)

I agree. The row gauge is usually not as important as the stitch gauge but you really need a good 4-6 inches length to determine stitch gauge. Also good to cast on more sts so that you can measure over the center 4 inches for the stitch gauge.
Yup, go up a needle size or two if you have too many sts in 4 inches. You don’t need to start the swatch all over again, btw. Just put in a few purl sts on the knit side and knit onto the larger needles.