I think you are making the best decision, to replace rather than try to repair your Apple Watch Series 4 which is now approaching 6 years old. It has done good service but it will be showing its age, and it seems like a better deal to go new with a new warranty and all new parts.
Your Series 4 is the oldest watch currently supported by Apple, so any more recent model will be an upgrade. Please go to the Apple Watch pages of the main Apple website (link) Watch - Apple where you will see the current range, and some online comparison tools to help your decision.