Is CyanogenMod easier to install in rooted device? I have failed to install CyanogenMod a few times in my Samsung Galaxy 4 (GT-I9605, Android 5.0.1 lollipop) in unrooted device. I still want to get CyanogenMod in my device so I am thinking to root my device and start the installation process 3rd time. 

Is CyanogenMod easier to install in rooted device than unrooted?
Both yes and no. You need custom recovery to flash CyanogenMod. If you have root with the non-recovery method, you can install the ROM manager from the Play Store, which makes the installation of a custom recovery rather simple. 

From there, you just download the CM zip and flash it. Note that the ROM manager does have install ROM as a feature, so you can install from zip and it should automatically reboot to recovery and flash (but I think it works with CWM only, as I have TWRP and it just reboot).