4% of some mass is 80 grams, so
(4/100) M = 80
Solving for M will give you the mass of the solution you require. If we assume that most solutions used in this medical context are mostly water, then 1 g of solution is about 1 ml of solution. If you need better accuracy, you will need to divide by the density of the solution (in g/ml).
Stephen La Rocque.