Unfortunately they might ask you to re-install Fairphone OS for that purpose. The Service Menu is only accessible within Fairphone OS.
There is an old FP2 app called “(Fairphone) Checkup” with a much more limited range of tests available on F-Droid and it mostly works on other phones than the FP2 as well, but I doubt that Fairphone Support will accept it for diagnosis on an FP4 (e.g. Checkup only includes tests for 2 microphones).