YIKES no need to harpoon them like that. Especially for a 4lb fish
You can...
a)gently place the scale hook in the lower jaw w/o puncturing it letting the weight of the fish hold it in place or
b) as Fast said, go through the gill opening but NOT over the gills. Either way is risky because pike are rambunctious during release and they can either tear themselves up or they'll tear you up with their 1/4" teeth.
I have a scale/ruler combo so it comes in handy.
