Stephan,
There are some examples that ship with CVI 5.5 (under /samples/daq)that implement a simple double buffered data acquisition. However I would suggest you contact our data acquisition engineers for more in depth information on how to acquire data from 4 SCXI modules since this thread is mostly for software related questions.
You can contact them at http://www.ni.com/ask.
Regards,
Azucena Perez
Product Support Engineer
Measurement Studio