I have lost all emails that are 4 weeks old and older on my ipad pro

When using the native Mail App, it is necessary to setup each mailbox/mail provider in Settings. After the account is setup, dependent upon the email provider, the available Mail settings may differ.

The majority of mail providers will use IMAP to connect to the mail server - and for these, messages are generally only removed from the inbox if you mark them for deletion.

Some mail providers, such as Microsoft (e.g. @outlook.com, @hotmail.com etc.) instead have a configuration option that will remove local copies of email, if not otherwise manually deleted, from the inbox after a configurable period - for which the default is perhaps 30 days. The local copies are removed from the inbox, but the messages should remain on the server.

For a Microsoft hosted mailbox:

Settings > Passwords & Accounts > [Account] > Mail Days to Sync - select No Limit to retain all messages in the local inbox

To reiterate, this configuration parameter is not used for all Mail providers.

I hope this provides some useful help and insight - and if your Mail provider is Microsoft or similar, resolves the problem.