Reason is an incredibly powerful music-making platform unto itself. This expanded greatly with the addition of the Rack Extension format for third-party virtual instruments and audio effects in 2012, and again earlier in 2017 with the announcement of VST plug-in support in Reason 9.5.
In this quick tip, synthesis guru Jim Aikin explains how to use a Rack Extension - a $25 one called CV Mutant - to crossfade between four audio sources. (Natively Reason provides for crossfading between two.) The sources are four instances of Dr. Octo Rex, each running a different drum loop, and the crossfading is quite smooth.
Another application would be to set up four different synths and use CV Mutant to morph between them, creating a very flexible "oscillator" that can change its character in sync with your beat and/or with whatever envelope you program in CV Mutant.
We're planning to see a lot more great tips from Jim in the future, so stay tuned!