Four inches of clear ice is usually safe to walk on. That’s clear ice, not milky ice. The clear ice is more stable. If it looks milky, the bonds between the water molecules aren’t as strong.
As you walk out onto the ice, drill test holes every so often. Check to make sure the ice is thick enough, and look for cracks or bands of white running through the ice. Also, check to see that the ice goes all the way to the surface of the water. On reservoirs and other bodies of water where the water level might fluctuate, the water might fall below the bottom of the ice. If that happens, you could easily break through the ice and fall into the water.
Even four inches of clear ice isn’t always enough. You can’t test the entire sheet of ice, and there could be impurities anywhere. If you fall through the ice and find yourself in near-freezing water, you won’t last long.
To survive, you’ll have to get out quickly. Never go ice fishing alone. Always go with a buddy. You might even want to keep a ladder in the truck. If you or your friend fall through the ice, the luckier member of the party can grab the ladder and help the other get to safety.
Don’t forget to pack extra clothes and hot liquids for every ice fishing excursion. You might keep a sleeping bag handy, too, along with a couple methods for starting a fire. If a member of your party falls through the ice, they’ll need to be warmed up quickly. The best way to do that is to strip them of wet clothes, stuff the victim in a sleeping bag, then take your own clothes off and crawl in with them. That’s why you want to go ice fishing with good friends.
Stay on top of the ice. But be prepared to do what you need to if someone falls through.