I have created one azure web app service (S1:1), I have registered custom domain (TXT, CNAME & A records in DNS) for this site. I am able to access site using FQDN (https://xyz.pqr.com) but when I am using IP address to access this site it give me below error.
Why am I doing this? I want to redirect another application to this application using DNS resolver for that it make use of IP address.

The
404error occurs due to multiple reasons, please check the below to resolve the error:IPaddress by using whatsmydns.netIPaddress.A recordandCNAMEconfigured.Make sure the custom domain is present in the Azure App Service like below:
Please check this MsDoc for few causes and solutions that may give you a pointer to solve the issue.
For more in detail, please refer below links:
Azure WebApps Custom Domain 404 error by AjayKumar-MSFT
Azure web app custom domain problem by Ryan Hill