Moved to iPhone Jailbreak. You'll need to start threads on this topic from mobile Safari rather than the app. The jailbreak and unlock forums are not accessible on the app due to Apple restrictions.
You can use Absinthe on a Mac or PC. Do you have any specific questions or concerns regarding jailbreaking? I recommend backing up your phone in iTunes so you have a clean non-jailbroken backup stored on your computer. Then do an iCloud backup and keep it set to iCloud for the duration of your jailbreak. If you have any problems jailbreaking, restore your phone to factory settings and try jailbreaking again. After the phone is jailbroken, apply your backup. I can give you specific directions for that if needed.
No, an iPhone 4S unlock is not currently available.