Alright, I have tried your suggested method by plug in the HDD one by one and my conclusion is that SATA port 5 & 6 has stopped working in Synology. So I attached the four HDDS to port 1-4 and after that they were all detected by the OS. The only problem was the console was showing error that 1 HDD has stopped working, I opened up the the volume manager, and 1 HDD was in green status. it means that is not in the array anymore, something like corrupt.. It gave me option to restore the array because it detected some old OS system files, reboot and it will restore/fix it by itself. Currently by writing this message, it takes 484 minutes, after that we will see whats gonna happen.
*Edit*
Wow that was really scary, I couldn't add back HDD2 back to the degraded volume. the option was not there to add HDD2 while HDD2 was showing as a healthy drive. At that moment I was clueless what to do next, so I digg around the menu a bit and saw the option to add a Hot-Spare drive to the degraded volume. Turned on that option and saw HDD2 was listed there, checked the marker and now its back to the volume where it belongs. And now the volume is finally repairing/rebuilding itself.