Assistive Touch is not available in Watch Series 4
As per Apple Updates, Assistive Touch will be available after updating Watch OS 8 for all Watch series but Why it is not available in Watch series 4?
Apple Watch Series 4, watchOS 8
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AssistiveTouch is compatible with Apple Watch Series 6 and later, and Apple Watch SE. Make sure that you have the latest version of watchOS. For more info, see:
Use AssistiveTouch on your Apple Watch - Apple Support