Alog's mass is 5 kg. after burning, the mass of the ash is 1 kg. explain what may have happened to the other 4 kg of mass.

We know that the mass of the log before burning is 5 kg. After it is burnt, the mass of the ash comes out to be 1 kg. So there is a definite mismatch of 4 kg. When the log is burnt, there is the formation of smoke. The air and water present in the wood vaporises to reduce the weight significantly. The combustion actually leads to the formation of carbon dioxide that mixes with the air and also water. The extra mass thus goes into the formation of carbon dioxide and water. 
Two good answers already. True the hydrocarbons sugars etc burn to release (mostly) H2O and CO2 + other gaseous oxides eg SO2. Also there is a large amount of water in the wood which is boiled off when the combustion takes place.