Can I install Linux 4.9 Kernel in elementary OS 0.4 that supports bfq disk I/O scheduler??? If possible, can you please point me to that repo or guide??
Yes, you can.
If your system is 64-bit (like most new-ish computers), you would go to the "amd64" section and download three files:
If you put them in one folder, you can install them all at once with
sudo dpkg -i *.deb.
You should be able to boot into GRUB, select "Advanced options for elementary OS", and choose the kernel version.
Note: an upstream kernel might not work out of the box, depending on your hardware! Don't be surprised if it doesn't work.
Yes you can install 4.9, or any other for that matter...keep in mind you could run into issues with a different kernel.
sudo apt-add-repository -y ppa:teejee2008/ppa sudo apt update sudo apt-get install ukuu ukuu-gtk
choose your kernel > install