Then I started to take a closer look at MIME types. Per default, the MIME type(s) setting for RAR archive in Speed Download 4 on my machine was
application/x-rar-compressed, application/octet-stream. But I found a list with some more possible MIME types for RAR files. It finally worked again fine after completing the MIME type(s) setting for RAR archive in Speed Download to
application/x-rar-compressed, application/octet-stream, application/rar, application/x-compressed, application/x-rar, compressed/rar. Now RAR file downloads are again intercepted by Speed Download 4.