A 4mm bead is 4 millimeters from the hole on one side to the hole on the other side. This size is about 0.16 inch.
Standard strands of beads are roughly 16 inches long; a strand of 4mm beads yields roughly 100 beads. If there are no knots or stitches between each bead, a crafter can calculate that it takes 6.35 4mm beads to take up 1 inch of linear space.
A 4mm bead is slightly larger than a size 6 bead, which measures 3.7 millimeters. Due to their small size, 4mm beads are generally available in fairly simple shapes, most often in rounds and cubes.