Gamertag hasn't changed in 4 days

no but have you tried redownloading your account

Also is the name you are posting with now the correct one?

If your GT hasn't changed at this point you will need to contact official phone/chat support at support.xbox.com/contact-us

When you change your gamertag it can take a few days for it to show across all services. If it hasn't changed everywhere within 4 days then contact support for more help
https://support.xbox.com/contact-us

Hello, first it's a pleasure to be able to help you today.

My name is welcome to the community.

Hello James, I'm sorry you are having difficulties contacting Xbox support, however try to follow the steps below to contact Xbox support via chat.

Go to: https://support.xbox.com/en-US/contact-us/

See the bottom left part of the page and make sure that the page has the correct region that you are in (ex: United States - English for USA)

If not, click on it then select the correct region that you are in.

Check and confirm that you are within the available time of support under Contact times.

Click Contact us

Select the drop-down menu options that will match your concern.

On the page, see the provided support articles that may help you with your concern. Click See contact options if further assistance is needed.

On the next page, select either Chat online or Request a call option then follow or enter the needed details.

If your GT hasn't changed that long after purchase you will need to contact phone/chat support at support.xbox.com/contact-us