The gauge would be measured from side to side of square after you finish 4 rounds.

Without seeing the pattern I wouldn't hazard a guess that every 4 rounds would add 7 inches.

My usual gauge for a four round granny square using worsted weight yarn (size 4) with an "I" hook measures 5-inches across the square (side to side).

Hope this helps,

Chris

If you want to have your afghan turn out the size the pattern states, you need to crochet to guage. (I’m surprised the gauge instructions didn’t include how many stitches across made whatever measurement.)

When gauge is important, you need to do a small piece to test whether size of your stitches is the same as the person who wrote the pattern’s stitches. So you crochet however many stitches across the guage suggests, then do four rows, then measure your resulting square side to side and top to bottom. It should be the same as the measurements mentioned in the guage. If it is not, you need to either adjust how tightly/loosely you crochet, use a larger or smaller hook as appropriate, or adjust the number of stitches and rows you crochet to get the size afghan you want.