I did the 3" ice last year ended up pretty much soiling myself. really had to be careful where i walked. I'm a bigger guy too, and could hear (and feel) the ice cracking and sagging under my feet. mostly was in 4' of water , but when I moved to 6' it started to buckle. Packed up , went home and changed my undies.
A local forest preserve pond wont open to ice fishing unless there is 5" of solid ice all the way across the lake. When over deep water I'm still kinda nervous on 5".
I make it a point to stay off the snowmobile tracks, and try to fish with a bigger guy than me and follow him out.
I love ice fishing, but I'm an ice wuss too. The Fox chain by us for being a lake/river system has a lot of current through it. Was out one time last year we had 17" of solid ice out of the current area, but as you got close to the current it got down to 4-5" real quick, maybe within 50' of the 17" of ice.
So 4" I'll do but won't really enjoy it. prefer 5-6 or better to actually have fun out there.