Count over a measured distance
Gauge tells you how much space your stitches take up. A pattern might ask for 20 stitches and 28 rows over 10 cm. Measure the width and height of an area inside a gauge swatch, then count the stitches and rows within that same area.
For example, 24 stitches across 12 cm gives 24 ÷ 12 × 10 = 20 stitches per 10 cm. If 36 rows measure 12 cm high, the row gauge is 30 rows per 10 cm.
Keep the span with the number
Write down both the count and its measurement span. Four inches is exactly 10.16 cm, so a result per 4 inches differs slightly from a result per 10 cm.
More stitches in the same width means smaller stitches; fewer means larger stitches. Needle or hook size is a starting point. Check your own swatch against the pattern before relying on its finished measurements.