I would take 2# off by sanding the sides, and then after that was done, I would finish by sanding the traped surface. Go SLOW and Stay Consistent. I just got done taking 7# off of a longbow by trapping it. It had a slight trap to the belly, and I ground @30degree trap on both limbs from the fades to about 6" from the tip. I was going to refinish the bow anyway, so I sanded the back and belly lightly as well as rounded all the glass edges. I couldn't be more pleased. It is a better shooter now to my non scientific "feel". 4# should be pretty easy. BTW I would stay away from the last 4-6" of the limbs when you reduce it.
Get to it!!!