I would assume the 3K kit makes it fuel somewhat like a 24v, but how about the 4K kit? Does this many RPM's require different valve spings? Is this really more for racing?
With the 4K spring kit you definitely need to do 60lbs springs on both the intake and exhaust valve springs. The BD 4K spring kit comes with one set of springs but both are recommended.
What are the benifets of revving to 4K? How does that work with the shift points of the transmission?(auto)
The benefit is more fuel and power over a broader RPM range, lots of people think we mean engine RPM but it is the pump that gets the RPM governor increase to 4K, which will usually relate to 3600 engine RPM.
It also helps to be running a bit more fuel than stock, so you can get to those rpm's. I had a 4k kit on my old Dodge, it really helps an auto. I found that it would rev to 4200, but it wasnt making any power past 3800. Then there's the, "It's there if you need it" scenario. Most of the time they dont need to go that high. Sled pulling is where it needs the revvs. Drag & street racing, it'll be good to have, but shifting before 3200 will make the best times. As said before, replace all 12 valve springs with the 4k kit. The valves start to float around 3400. A GSK is one of the best mods to do on a 12V. I also have a poor man's GSK by using a couple 3/8" washers. Some injector pumps act differently than others when doing this though. Some guys report it idles at 1500 after doing the mod, while some get great results. Click the link in my sig for more info.
