SoundSoap is installed as an Native-64 AAX (ProTools 11) and RTAS (ProTools 8/9/10) plug-in. Avid discontinued AudioSuite plug-ins and as such, SoundSoap 3/4 is not available in the AudioSuite format. You will find SoundSoap listed in your ProTools mixer plug-in inserts under "Noise Reduction".
Update: SoundSoap 4 works in ProTools 12 as both 64-bit AAX and AudioSuite. You will find SoundSoap listed under the AudioSuite menu in ProTools 12.