New sensors Apple Watch after series 4?

Apple Watch Series 4, Series 5, Series 6, Series 7, Series 8, or Ultra2 also have built-in electrodes in the Digital Crown and the back of Apple Watch, which can measure the electrical signals across your heart when used with the Heart Rate app or the ECG app. When you place your finger on the Digital Crown, it creates a closed circuit between your heart and both arms, capturing the electrical impulses across your chest. 

For more info, see:"

Monitor your heart rate with Apple Watch - Apple Support

Please be aware the approvals for some countries for Apple Watch heart monitoring functions do not cover use with an existing diagnosis or for monitoring a known condition. See Apple's notes in Heart health notifications on your Apple Watch - Apple Support and Track your AFib History with Apple Watch - Apple Support

My advice would be to consider upgrading your Apple Watch Series 4 sooner rather than later because it will be getting to the stage where a new battery will be needed, and the improved functions on the later models are amazing. If it was my choice I would probably try to hang on for the new models which are usually announced around September, but that will have to be your decision. Even now the current range offer a great choice. Hit the Apple website and online store (links at the top of our pages here) to see what the current range can do.

Third-generation optical heart sensor

They did add Blood Oxygen sensor after the Series 4.

They have added a temperature sensor.

Crash Detection (as in automobile crash)

Improved the compass

Always on Display

Sleep monitoring

I'm sure I missed something, but those are the highlights.