Apple Watch Purchase

There are not a lot of physical differences between the Series 4 and 5, so it was an easy decision to switch the Series manufacturing equipment over to making Series 5 watches. There are differences, and here is my list:

Apple Watch Series 5 vs 4

• Always on Display using a variable rate refresh that can go down to 1 second intervals (A low-temperature polycrystalline oxide (LTPO) display is a special kind of backplane technology designed for OLED screens and developed by Apple).
• Compass (of course your iPhone has a compass too).
• Cellular Apple Watch has emergency calling when away from your cellular provider (international travel), or if you do not have a cellular contract for your Apple Watch. 
• 32GB storage vs 16GB
• Titanium and Ceramic cases available
• S5 CPU on a Chip vs the S4 CPU 
• Series 4 is no longer manufactured, and new in the box comes with watchOS 5 vs the Series 5 comes with watchOS 6 and requires an iPhone 6s, SE, 7 or newer running iOS 13.

No the series 4 has been discontinued,only the watch 3 and 5 are available to buy from apple.