Only use a 4.1 bit if it specifies to do so, otherwise use a 3.6. I would use a 3.6 in this case. It refers to the diameter of the bit not the depth, both 4.1 and 3.6mm bits will be the same depth. If you bubble the base its because they arent the right length screws (ie forgot the riser plate, non standard screws etc...). Use a 3.6 only use 4.1 on skis with metal in them or if they specify.......