This is a difficult question. The number is not as important as breathing difficulty, the duration of the problem, whether the problem is getting better or worse.
It is impossible to answer the question about a 4 year old specifically. All of the above are related to the answer. I would speak to your physician and address your concerns directly.
The only way to KNOW an indiviaul's oxygen saturation rates is by testing with either an arterial blood gas (requiring lab analysis) or a pulse oximeter (firefighters, EMTs, medical centers & some physician's offices have these medical devices).
In general, oxygen saturation levels should be at or above 90% all of the time & normally at sea level, they are 95% or above.
Except during acute attacks, most asthmatics have normal oxygen levels, so that's not a reliable indicator of how well controlled asthma is and whether hospitalization might be required.
When a particular individual needs urgent or ER care for asthma is something that should be worked out in advance between the patient & physician in a written Asthma Action Plan. Below is a site with good info about this, but a plan needs to be developed & updated on a regular basis to be effective for the patient. Please discuss this further with the physician treating this child.
http://www.njc.org/disease-info/diseases/asthma/living/tools/action/index.aspx
Starion