Does He Hahve Asthma?

My son has just turned 4 years old and woke up with another cough yesterday morning, no cold with it just what sounds like a chesty cough. This will the the 4th one in 6 months, I was wondering if this could be asthma? I know it might just be a case of him having another cough but why does he keep getting them like this? We are not smokers, and there is no smoking in the house at all not even visitors.