Few climbs are constant in pitch. They vary as they go. The steep parts can still be pretty tough even though the overall grade is not too severe. Where I live, there are no mountains. There isn't any truly flat land either. I guess you would call it rolling countryside. We have no long climbs but plenty of short ones. Some the short ones can be pretty tough.

There is a town 6 miles west of my farm that is about 600 ft. lower in elevation. That's a very mild pitch. Almost barely discernable. You can calculate it. In a car it seems pretty flat with some rolling hills. But it is a pretty steady pitch and I can tell you the difference between riding there and back is really meaningful. I can get there significantly faster than I can get back. It does make a difference.

I drove through Tennessee and Georgia just about 5 weeks ago. The drop from the top of the ridge South and down into Chattanooga (just north of the Georgia border) is marked as 6% so it is in the same ball park as the climb you are talking about and a whole lot steeper than the trip to North Liberty near my farm. They have escape roads for trucks and warnings about using lower gears to descend. I'll bet I would be dog tired after climbing from Chatanooga to the summit of that ridge for several miles. I know I would be. It might be a piece of cake for a pro racer but It is nothing at which to sneeze for we mere mortals. I think you'll feel every foot of that 4% pitch.