4" is that all? I think the record thickness on my IG was around 17" with around 9 being average. I weight down a couple of 5 gallon Home Depot pails with a smaller bucket inside full of stone so they just float, then let them freeze in place. Pull out the stone bucket then yank on the handle and the buckets pop out leaving a hole in the ice. Measure the thickness through the hole and also drop a line in to see what I can catch. So far nothing.
Al