 A repeat-dose open label randomized incomplete block design in pediatric subjects with asthma, ages 4-11 years, to compare systemic exposure and pharmacodynamics of fluticasone propionate and salmeterol following ADVAIR HFA 4521mcg2 inhalations, ADVAIR HFA 4521mcg2 inhalations with Aerochamber Plus Spacer and ADVAIR DISKUS 10050 twice daily 