That engine makes its peak torque at 3,250 RPM. It makes peak HP at 4,750. To climb a hill in a hurry you need more HP. So, by staying below 4,000 RPM you are giving up HP that the engine could be making if it was spinning faster at 4,750. However, many people say the "best" RPM to pull a grade is at peak torque. At that RPM the engine is most efficient at hill climbing but not necessarily the fastest. So, take your pick, you can have speed with RPM or you can have efficiency and possibly cooler temps at peak torque but you can't have both.