Not quite J.
redsn0w is prefectly capable of jailbreaking the 4S too, as per our sticky -
http://www.iphoneforums.net/forum/iphone-jailbreaking-17/jailbreaking-methods-iphone-18519/
london29 - In terms of "safest" there is no concept of an unsafe jailbreak in the iPhone world just to put your mind at rest!
Both redsn0w and Absinthe use exactly the same method, which is tried and tested, and both are equally as "safe". You should not normally run into any problems. If you do, and quick restore and re-jailbreak will fix it.
Also, it's important to point out that 5.1 is not the current version of IOS. 5.1.1 is, and that is the version you should use. It's only a very minor point, but important. You won't be able to install / restore 5.1 on your 4S.
Finally, after IOS6 is released later today you will not be able to restore 5.1.1 period. Your only option will be to restore to 6.0 (as things stand) and there will be no jailbreak on the 4S on 6.0 for a while (many weeks, maybe months IMHO). So if you are going to jailbreak, do it on 5.1.1 and do it today!
That said, Happy jailbreaking!