Ignoring the ammeter for a moment, lets first assess the condition of the battery, the alternator, and the voltage regulator:
What's the voltage across the battery terminals with the engine off?
What's the voltage across the battery terminals with the engine at idle?
What's the voltage across the battery terminals with the engine at 4000 rpm?
Now please explain is the ammeter reading full scale left, or full scale right? Max discharge (left), or max charge (right)?