Why is the number of trophic level restricted to four or five in a food chain?

All organisms need energy to survive. It requires many units of lower trophic level organisms to feed the next trophic level. For example many first order consumers (fish) are required to feed one crocodile (top order consumer). This is because energy is lost as heat at each trophic level, so the energy available to the top feeding level is far lower than that of the producers (plants). This is what restricts the number of trophic levels in a food chain to four or five.