How do I unban an account that is about 3-4 years old since banned?

If you're banned permanently there's nothing you can do

You can't. Any accounts that have been permanently banned, instead of temporarily suspended, cannot be recovered at all. Bans and suspensions are not something Xbox Support or community support (like the Xbox Ambassadors) can assist with.

While you can't use that account's log-in information to log-in to Xbox Live or any other platform that requires a log-in from a Microsoft Account, you can still view the FAQs about the difference between suspensions and bans on the Enforcement Website without
needing to log-in.

https://enforcement.xbox.com/en-US/home/howTo#sub-topic0-2

https://enforcement.xbox.com/en-US/home/howTo#sub-topic0-3

The only way to check the details of the account's ban would be by trying to log-in to an Xbox or a Windows Device of some kind, it should give you an error message and you'll have your answer. Please keep in mind, again, bans are not reversible.

Here's some information for future reference that could prevent bans/suspensions from happening again: https://www.xbox.com/en-US/legal/community-standards

You can’t unbanned your account if it’s been banned for four years then it’s been permanently banned for violating Microsoft teams and conditions of the system or something along those lines

Not this account got ban it my old account is permanent is banned about 3 years go and this my new one and it got ban for 4 days for trash talk then I realise what I doing wrong then I stop doing it

You would have had to contact the enforcement team at the time that the ban happened. Since it's been over a year, there's no way to contact them about the ban and it will not be unbanned.

Your warning is the code of conduct/terms of service you agreed to when you signed up. The rules were exactly the same 4 years ago.