necdetdoni,
When you installed 4.0C, which version did you install?
A. If you installed 4.0C STANDARD (with new sounds ... using more than 10 FD for the full operation or using KetronFT), and you want to go back to 3.0b or earlier and also loose the new 4.0 sounds, you will need to install the OS3.0 Diskette. When completed, go to the HD - FOLDER 90 (reload) and select the SD1Reload.rld file and press F2 (LOAD) to load the file.
B. If you installed 4.0C ORIGINAL(with new sounds ... using only 3 FD for the full operation or using KetronFT), and you want to go back to 3.0b or earlier, you will simply need to install the OS3.0 Diskette.
Hope this helps.
Thanks,
AJ