Hi @cyphus, thanks for you query. Indeed the Tobii Pro SDK has a number of functions that can detect user presence, eye trackers status, etc so if you have already purchased the special licence to use the 4C in conjunction with Pro SDK then there should be no need to also use the Core SDK unless you are intending on developing for interaction applications for which the Core SDK is uniquely specialised. (Although the functions you asked about are indeed in Core SDK as well)
In case you were wondering, we do not recommend using *both* SDK in a single solution as this can lead to issues furter down the line.
For specific questions related to the Tobii Pro SDK please contact Tobii Pro Support @ http://www.tobiipro.com/contact/contact-support/
Please let us know if we can be of any further assistance.