Tony drank yesterday 100% - 4% = 96% of what he drank two days ago.
Since 96% is 48 ounces, then every 1% is equal to 48/96 = 0.5 ounces of water.
So, Tony drank 0.5 x 100 = <<0.5*100=50>>50 ounces two days ago.
#### 50