I've never got why people get so hung up about "spin-time".. LoL.
If you NEED to get spin-time then get some new bearings, strip them of all grease and leave them without lube. Or better still, treat them with a surface friction reducer (Eg. Pro-gold). Your bearings will spin forever.
Of course, take care when you re-install your bearings to make sure things are properly aligned. Also, i've found that replacing the o-rings in the centre drive cups with foam helps.
BUT!! spin-time is less important when running under load. So, therefore it is more beneficial to actualy lube with a light-ish oil. This will slow your bearings down - and with the car unloaded without wheels etc. may seem to spin less.
But, once the car is up to speed the heavy oil is thrown by centrepetal force and you are spinning free all the same.
The Pro4 is a very smooth and free car out of the box. You would gain much more from working on set-up and driving technique than focusing too much on "spin-time"!!
Incidentally, the lighter your drive-train is (eg. upgraded with Ti or Alu shafts, delrin etc.) the less momentum your car will have and therefore less spin-time.
Some ppl measure spin WITH the wheels and you can get some stupid long times. Without anything on the car, i'd consider around 3s very good indeed.