This error will not affect those who have not updated BBC SO Discover or who installed BBC SO Piano.
This issue is also fixed inside the installer from July 4th, so it will not occur going forward.
Normal BBC SO users, who updated BBC Discover before July 4th to V1.7.0 may have an Error 4. This occurred due to a missing IR sample file called "MV1_IR1_01.spitfire" in the BBC Discover part of the install.
You can now reset and redownload BBC SO Discover to fix this issue. This link will show you how to redownload a library.