Well the series 4 went on same September 21, 2019, so if you bought yours at launch it should be just over a year old. Did you not get AC+ with it?
I'm not sure they do screen replacements at all. The reason I say that is I cracked my S3 a few weeks ago but have AC+ on it until March 2020. I started a chat session with Apple, added some photos of the watch, and they shipped out a replacement. When it arrived I put my old one in thew same box and sent it back.