Are you seeing that error in the GUI or SSH? If SSH this is a common problem after flashing because when you connect via SSH there is information stored in a “known_hosts” file. More info here: Fix "Warning: Remote Host Identification Has Changed" Error
Assuming you aren’t managing other remote hosts with SSH keys then just delete everything and save the file. Or if you want to be more careful about it identify any line starting with 192.168.4.1 and delete those lines. Save the file and then you’ll be able to connect again.