From that statement, it doesn't seem you have any familiarity with the Linux filesystem hierarchy. To get an understanding of it I would suggest you read the information at the link below and/or do an online search for Linux filesystem hierarchy. That will explain where the /etc and grub folders are.

http://www.tldp.org/LDP/Linux-Filesy...ierarchy/html/

If you have an EFI system, you can find that out from Ubuntu/Fedora by simply running the command: efibootmgr
If it gives you output, it's EFI.

You haven't posted any information on your actual drive/partition setup so it's not possible to say what you might be able to do.

I mean both Fedora & Ubuntu have /etc/../grub files folders...

from this two grub locations what is responsible for bootloader and choose one OS from the 4 ...???

Generally any files within /etc/ are ignored by the actual bootloader. They are configuration files used by the commands/utilities which update grub. For example, when you run "sudo update-grub" in Ubuntu, it will read configuration files in /etc/grub.d in its process of updating the grub files which are actually used by the bootloader.

The bootloader will use files in /boot/, including /boot/grub/. Which one, though? It depends on which boot partition the bootloader in the MBR is pointed to. Assuming you have not manually run something like "sudo grub-install", then the MBR will point to whichever OS you installed last.