If it is solid blueice (clear, cold & no air bubbles)4" is safe for a single person on foot. If the ice has small air bubblesdouble it to at least 8".
Durring very cold times ice cracking sounds that travel accross the ice are signs of ice expansion due to frezing a generally no problem. Just ensure the crack does not expose open water.

Be very carefull in areas of current. (rivers, narrows ect.)

Black coloured ice indicates melting - Stay Off

If you don't know the area stay to the areas others are using.