The liver breaks down ethanol with an enzyme…

Step 1. After 4 h. $0.75\ \text{g/L}$, because $1.35\ \text{g/L} - 0.15\ \text{g L}^{-1}\,\text{h}^{-1} \times 4\ \text{h} = 0.75\ \text{g/L}$.

Step 2. To $0.30\ \text{g/L}$. The level must fall by $1.35\ \text{g/L} - 0.30\ \text{g/L} = 1.05\ \text{g/L}$. The time is $\dfrac{1.05\ \text{g/L}}{0.15\ \text{g L}^{-1}\,\text{h}^{-1}} = 7\ \text{h}$.

Step 3. Why not straight to zero. The enzyme works flat out only while there is enough ethanol to keep every enzyme molecule busy. Below about $0.2\ \text{g/L}$ some enzyme molecules wait for ethanol, so the rate falls with the level and the order changes.