Does apple replace cracked screen’s on series 4 watches.

I'm not sure why opening a link is confusing, but click these blue words --> watch Service Pricing - Apple Support

Inside that link you will see that a replacement Apple Watch 4 will cost the following:

Apple Watch Series 4 (GPS) Aluminum $ 299

Apple Watch Nike+ Series 4 (GPS)$ 299

Apple Watch Series 4 (GPS + Cellular) (Aluminum) $ 349

Apple Watch Nike+ Series 4 (GPS + Cellular)$ 349

Apple Watch Series 4 (GPS + Cellular) (Stainless Steel) $ 399

Apple Watch Series 4 Hermès$ 399

Schedule an appointment at your Apple Store Genius Bar. Take your watch and iPhone with you.

For repair & service:

watch Repair - Official Apple Support

watch Service Pricing - Apple Support

Chances are that instead of Apple fixing it, they will send you a refurbished replacement.

Click the links javaliga provided (words in blue).