The row gauge is usually not as important as the stitch gauge as most patterns have you knit for X inches not X rows. So CO about 24 sts and knit for about 3-4 inches, then measure the sts in the center to see if you get 4 per inch.
Do the sts (4 sts/1") for the medium actually measure 46" or less or more? Measure a sweater you have and see what it is at the largest part, usually just under the arms. If it's close to the measurement for the medium, go for that one. Measurements are sometimes given for finished size of the sweater, sometimes they say `to fit X"', so you can compare the sts and figure out which this is.