I have the same problem. I am not sure if it is possible!
On my system its even when the PC is switched off completely unless disconnect the power to it. I guess its like that by design to allow USB charging devices.
Yes S3 is what Windows use normally. Its almost like a complete shutdown (S5). All the lights switch off, fans stop, hard drives switch off....etc. The only difference from shutdown (S5) is when you wake up the computer it goes straight into the login screen. Also When you login you find all the windows/programs that were open are back again exactly as they were before sleep. Unlike shutdown the PC reboots completely and go through post...etc. S4 is only when you have Hibenation enabled which is not necessary any more with SSD drives as S3 now does the same thing. I disabled it completely by "powercfg -h off" which deletes the hiberfil.sys file from the C: drive and save more than 3GB. In all cases on my system USB power still remains on and I don't have "enable USB Power in S4/S5" in my bios I just have I just have "Resume From S3/S4/S5 by PS/2 Mouse/Keyboard" which I use but thats a different thing.