Im on 30 mg of steroids for 4 days then stepping down for an asthma flare up, will this cause a big hit to my immune system?

Not particularly: This is a standard short term burst treatment and often used in asthma management. An occasional short burst therapy will not alter your immune system. Taken daily for a month and it likely would.

Unlikely: Short courses of steroid have little impact on your immune system but essential to get you out of asthma exacerbation. If you need frequent steroid Rx for your asthma even while on a controller drug, need to consider avoid triggers and even addition of new biologicals.