Listed below are the steps to install iOS 8 GM build on iPhone 4S. Please make sure to backup iPhone 4s before executing the below mentioned steps.
Step 1: Login to developer.apple.com and access Member Center
Step 2: Navigate to iOS section under Dev Centers
Step 3: Select iOS 8 GM Seed then click iPhone 4s under iPhone section. This should start download the iOS 8 for software for iPhone 4s on your Mac
Step 4: Once the file in downloaded and extracted on to your system (ios_8_gm_seed__iphone_4s__12a365.zip), Launch iTunes, navigate to iPhone 4S then click Check for Update holding option button on your keyboard.
Step 5: Select the iOS 8 restore for iPhone 4s and click Open. This should display the following message on your iTunes screen.
Step 6: You should notice Extracting Software, Preparing iPhone for Software update and Updating iPhone software messages.
Finally your iPhone 4s will be upgraded to new iOS 8.