My 21mo daughter gets high fevers when sick. Temp gets bt 103-104 she's still pretty happy, but much higher, she's lethargic until meds work. Worried?

Typical. Some patients are particularly sensitive to different parameters, such as temperature, blood pressure, blood sugar, etc. This is an example of how if you have a derangement of one of them enough, it causes major problems. If she is lethargic for any reason, she needs to be seen, either at her physician or an emergency room.
Normal. The effect of a given temperature depends on age and the maturity of the nervous system. Many kids tolerate 102 like an adult would do with 100, but most will lose it at over 103-104.That is pink elephant on the wall territory for kids while adults might at 102. Lethergy with fever is a normal body response, it turns off the lights to conserve energy, so to speak.