How can you estimate lbs of pressure?

Pick up a 1-pound can of beans and sense how much force it takes. That's an easy way to calibrate how it feels. That sounds like a lot of force for anything but a Hackamore on a very stubborn horse. I seldom use more force than a few ounces.

You must get your terminology correct. You cannot pull on a rope and express the action as a pressure. If you pull on a rope you apply a force. The correct unit to express force in the SI system is the Newton.

In some older systems force was expressed in pound units, but this is very unscientific and not precise. But if you think this way and merely want some estimation (as you have stated) then I suggest you obtain a cheap spring scale graduated in pounds and attach it to a heavy object. You can then pull on the hook and read of the "force" you are applying in pounds. This will give you an estimate of what a 4 pound "force" feels like. But this is hardly science.

is the rope attached to an object? what does the object weigh? is object on a flat surface or an incline (pulling up or down)? is the surface frictionless, low friction, high friction?

need more data to answer regarding the effect of 3-4 pounds force applied to a rope.

google ^^