how do I reset myold apple watch 4 to cear all my personal information

Unpair and erase your Apple Watch - Apple Support

Or;

To unpair and erase watch and remove Activation Lock.

On a computer, go to iCloud.com and sign in with your Apple ID.

Go to Find Iphone.

Select All Devices, then click your Apple Watch.

Click Erase Apple Watch.

Select Next until the device is erased.

Click the remove button next to your Apple Watch.

See > https://support.apple.com/en-us/HT204568

You’re welcome, but (a clarification) we’re not Apple Support. We’re other users, like you.