Question

Is Comodo / Sectigo compatible with legacy devices like Android 4.x?

Answer

It depends on a certificate chain used.
Some prefer legacy devices. They are recommended to use the old chain.
As of 30-05-2020 such devices without a new root certificate in the list of "trusted" will show an error.

If the device detects HTTPS as non-trusted on legacy Android device, then install additional intermediate. Installation guides are here. Find additional intermediates here.

Comodo rebranded into "Sectigo" on the 01-11-2018.
Since the 14-01-2019 major part of Comodo, products changed root certificates.
Products active after 14-01-2019 use new root during activation and reissue.
Products active before 14-01-18 use old root for activation and will use the same during reissue.

Most browsers support the Addtrust External CA root certificate, so Sectigo continues offering this root certificate. New customers are already using the Sectigo RSA CA root.
At the time of the expiration of its relevance, users already have Sectigo RSA CA root represented in the trusted root certificates, it will be used automatically.
Installing the old root will not lead to problems until 30-05-2020.

Certificates are issued for a maximum of two years.
Certificate may have a longer validity period than the root itself that you use due to "cross-signing" technique that does not cause no problems.