Keep the measured span
A gauge of 30 rows per 10 cm gives 3 rows per centimeter. A piece of 60 rows at that gauge measures about 20 cm. A pattern specifying 4 inches uses a slightly different span: 4 inches is 10.16 cm, not exactly 10 cm.
Why it matters
Matching stitch gauge does not guarantee matching row gauge. Different row counts change the length of sections worked for a fixed number of rows, and can affect shaping placed at set row intervals.
Wash and finish a gauge swatch as you plan to care for the project, then measure inside its edges. The stitch-gauge calculator gives separate results per 10 cm and per 4 inches.