Based on Saucony women's footwear size chart, women's shoe size 4 in centimeters is 22.5 cm.
Zappos.com shoe-size-conversion
If you selected the gender or age from their drop-down menu followed by the Shoe Size & specify inches or centimeters, size 4 is 20.8 cm and size 4.5 is 21.3 cm.
Nike Women's Shoes - Size Chart
Nike CM size displayed on shoe boxes and labels is different than Foot Length (cm). Their US women's size 4 is 21.1 cm and size 4.5 is 21.6 cm.
Astarshoes Size 4 Women Shoes Length
Astarshoes size 4 in centimeters is 22 cm. If you purchased small size shoes from astarshoes before, you can check their shoes here. Or if you do not know your size, use the How To Measure Foot Length on size guide to help you find the right size.