In a particular food chain, if the energy produced by the autotrophs is 1000 Kcal, then the amount of energy which reaches fourth trophic level is:
0.1 kcal
Basically, everytime an organism consumes something, 90% of the energy is lost as heat, in digestion, etc. So, if you start with a plant (which is an autotroph, meaning it makes its own food) at 100% energy, the rabbit that eats it will only get 10% of the energy from it. Then the fox that eats the rabbit will only get 1% of the energy from the plant. This continues down the food chain. This is the basis of 10 % law introduced by Raymond Lindeman (1942). According to this law, during the transfer of energy from organic food from one trophic level to the next, only about ten percent of the energy from organic matter is stored as flesh. The remaining is lost during transfer, broken down in respiration, or lost to incomplete digestion by higher trophic levels.