Why do food chains rarely have more than four trophic levels?
Because so much energy is lost to life processes.
As you progress through each trophic, each organism on that level loses energy gained from the previous trophic. This is due to losing energy through wasted material (e.g. bones) and to life processes, 7 of which can be remembered by MRS GREN:
This means that each time, energy is wasted and lost from the food chain. More than four trophics would result in so much energy being lost from the initial trophic (producer), that it would be largely pointless! This is shown very well in the diagram below:
I hope this helps; let me know if I can do anything else:)
Sources cited in comments.