The engine has a finite amount of power to produce. You can increase or decrease torque but you cannot increase power from gearing. The delicate balance between torque and RPM dictates dictates exactly how that power is applied. At the wheel you either get high torque and low RPM, low-torque and high RPM, or somewhere in between.

What you are experiencing at the beach is that you need wheelspeed to move around and that 4HI is giving that to you.