A 4" square is a decent size. But I usually determine my swatch size depending on the gauge. For example,
this yarn's gauge is 4 sts = 1” #8 needles (assume stockinette). So I would cast on 14 sts and work stockinette over 4 sts with a 5 st knit border on each side. The border pops out the stockinette and makes it easier to measure. Just one humble person's technique...
