Q: Use my iMac to make Facetime video call to 4S iPhone in Israel

It should be possible as long as Israel allows the use of FaceTime. Some Middle East nations have banned the use of FT.

Both devices must first be configured to use FT. You would register an email address to use for the Mac and the phone number for the iPhone. Once these are registered with the Apple FT servers it is fairly simple to place a call.

Yep. Read here:

http://support.apple.com/kb/HT4319