Thor - Gods Own Synthesizer
If god used a synthesizer Thor would be it. This beast of a synth is to large to fit in one screen 1024x786 screen. At first glance it's mind blowing. The Thor polysonic synthesizer features six different oscillator types and four unique filters. Sounds cool right? Properllerhead claims its an unstoppable monster of a sound generator that utilizes synthesizer technology from the last 40 years. Six open filter and oscillator slots let you load up three different synth filters and three separate oscillators simultaneously.
RPG-8 - Monophonic Arpeggiator

RPG-8, a brand new unit dedicated to the art of arpeggiation. Turn boring chords into fluttering melody lines with the RPG-8. Although very hands-on and user friendly, the RPG-8 boasts some very advanced features under the surface. Features that will change how you play Reason's instruments. The 'Manual' mode will arpeggiate notes strictly in the order they were input, for realtime arpeggio control.
ReGroove Mixer - Realtime Groove Console
The ReGroove mixer in Reason 4 gives you more than just a set of sequencer swing parameters - this is a unique device dedicated to one thing: the groove. You can lock all your tracks together into one unified feel. Or you can apply different settings to up to 32 musical elements in your song, for ultimate control.
Sequencer - Major Srugery on the Sequencer
With a whole new look, a ton of fresh features and a completely new way of handling sequencer data, the Reason sequencer has matured. Dedicated to turning your ideas into great music, the new sequencer is swifter, stronger and more intuitive than ever. The key word here is workflow.