It is possible to video conference between a Samsung Galaxy S and an iPhone 4 but only by using third party software. Skype is a free download in both the iOS App Store and the Android Google Play store that allows two devices connected to a Wi-Fi network to video conference with each other. The two Skype users would have to have each other's usernames for the service.
There are also other free apps like Tango that allow both users to video call as long as the application is downloaded and installed on both phones. Apple provides its own video conferencing software called FaceTime on all iPhones, but this software can only be used between iPhones, iPads and computers and is not compatible with Android devices.