If you need to plant 4000 acres, you need to keep your speed up. If you are just looking to plant a little farm, you can drive slower, do a better job, and will have plenty of hp to get the planting done.
It does start to be a problem controlling a bigger planter on the road, takes some real weight for you to not feel like a tail wagging the dog with a fully outfitted 12 row folded up going down the highway. Probably much more of a concern than actually in the field at that point.
Friend of mine runs a 8 row 7000 with a 7 row 7100 behind it for 15 inch beans. No fertilizer, but pulls the rig with a JD 4020.