Final answer:
The ideal defibrillator to use for a 4-month-old baby in cardiac arrest is an AED with a pediatric dose attenuator.
Step-by-step explanation:
In the case of a 4-month-old baby in cardiac arrest of unknown cause, the ideal defibrillator to use would be an AED with a pediatric dose attenuator.
This type of defibrillator is specifically designed for use on infants and children and adjusts the energy and waveform of the shock to suit their smaller size and physiology.
It is important to use a defibrillator that is appropriate for the age and size of the patient to ensure the safest and most effective treatment.