You will need to perform the operation via SQL*Plus using the ALTER DISKGROUP command. This error is caused by an intentional Oracle ASM validation check that enforces uniform disk sizes within an existing normal redundancy disk group where all current members are equally sized. The solution is to explicitly instruct ASM to use only a portion of the new disks that matches the size of the existing 4TB disks by adding the SIZE clause to your command. You must specify a size that precisely matches the TOTAL_MB of your current disks (e.g., SIZE 4096G), which will effectively make the new disks appear as 4TB to ASM, thus satisfying the requirement for uniformity. For proper redundancy, it is also critical to add each new disk into its own new failgroup. This specific operation bypasses the ASMCA GUI limitation and allows the disk group to be extended successfully.

FYI: Based on the error message and your description, the issue stems from a specific feature introduced in Oracle ASM version 12.1.0.2. This feature enforces a check that prevents the addition of disks of a different size to a normal or high redundancy disk group if all the existing disks in that group are of an equal size.