I have a collection containing machines to be patched. I set collection specific restart settings by using modify collection settings for that collection. Restart countdown is of 4 hours with final countdown of 1 hour. I created deployment containing set of patches to be deployed on that collection. I checked option of supress reboot server and workstation. What will be the reboot behaviour?

If you chose to supress reboot then no automatic reboot will occur.The restart countdown is only used if there is a reboot pending, which there will not be in your case.