1. To calculate the alcohol content of a 500ml beer after 4 hours, it is necessary to know the initial alcohol concentration of the beer and the metabolic rate of alcohol in the human body. The initial alcohol concentration of beer is 5% (50 degrees), and the metabolic rate of alcohol in human body is per hour 15ml.

2. The absorption and metabolism of alcohol by human body can be expressed by the following mathematical model: alcohol content (A)= initial alcohol content (C0) minus alcohol metabolism (M)× time (t), where alcohol metabolism (M)= 15ml/h and time (t)=4 hours.

3. Substituting the known values into the formula, we can find out the alcohol content after 4 hours. The results show that the alcohol content is negative 59.95% after 4 hours, and the alcohol content of a bottle of 500ml beer is negative 59.95% after 4 hours.