By general rule, we have that on each trophic level only 10% of the energy passes to the next level, so if producers generate a 1 000 000kcal the primary consumers only will get 100 000kcal, and so on so we can solve it in the following way:
First level --> FProducers: 1 000 000kcal
Second level --> primary consumer: 100 000kcal
Third level --> secondary consumer: 10 000kcal
Fourth level --> Tertiary consumer: 1000kcal
So now we can estimate what percentage our 1000kacl represents of the producers 1 000 000kcal
1 000 000 100%
1 000 x
x= (1000x100)/1 000 000 = 0.1%
Therefore we can say that at the fourth trophic level only 0.1% of the producers energy arrives.