UB,
The Med Board will undoubtedly be more complicated than any answer we can give you on here. However, the first step is through the AFCO and answering a few basic Qs, including asthma history. If you have been clear of symptoms and treatment/medication for 4 years, you will have passed that initial hurdle. You should then concentrate on getting to OASC and not worry about the Med Board; there is nothing you can do about your history!