I need a replacement battery for my IPad Pro 4

You can establish eligibility and cost of a battery replacement in your country/region here:

iPad Repair & Service - Apple Support

You might choose to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - prior to committing to a battery replacement or other repair:

Genius Bar Reservation and Apple Support Options - Apple

For practical purposes, in lieu of replacing the battery and returning your own iPad (a process that may take several weeks - as the iPad must be sent to a service centre), Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for the battery replacement.

You may wish to start with this link:

iPad Repair & Service - Apple Support