How long/far can the spectrum ST4 and Dana Dolly go?

The answer here is simply a function of the length/size of your Dana Dolly rig. The Dana Dolly Integration Kit comes standard with an 11.5 ft belt that is suitable for motion along a 10 ft Dana Dolly track. For longer tracks you can purchase extra belt by the foot here:

https://emotimo.com/products/xl-belt-by-the-foot

Please note that proper belt tension will be more difficult to achieve with longer track systems.  As you increase the length of the belt, the belt stretches and has elastic properties.  In short, 20 feet with careful planning is realistic for slower video moves.  50 foot moves with timelapse with moderate intervals should be achievable as well.