I need to write a digital oscilloscope type application. There are many great static graphing controls out there, but I need something that can graph 16 traces processing 4
You can take a look at QCRTGraph control. You can also look at this CodeGuru project.
Though, honestly, with your requirements you might be better off doing your own C++ implementation of it. With only 0.25ms per sample, locking and updating the buffer of any third party control is too expensive for you to afford.