So they spray fuel four times a cycle? Even on the exhaust stroke where it gets shoved out into the converter? Even on the power stroke after the plug has fired? Isn't that a huge waste of fuel? Doesn't it dump raw gas into the converter?
You're both correct and incorrect: Yes, in your particular setup each injector opens once per revolution. Intake stroke, compression stroke, power stroke, and exhaust stroke. For 3 of those 4 strokes, the intake valve stays closed and the injected fuel pools on the back of the intake valve, waiting to be inducted on that cylinder's next intake stroke.
As far as the rest of your concern: it's a simple matter of programming. In a sequential injection system, the PCM opens the injector for the amount of time needed to provide a single cylinder the necessary amount of fuel FOR A GIVEN INTAKE STROKE. In a batch fire setup, the pooled fuel on that's waiting on the back of the intake valve is taken into account.
For example: at idle, let's say a single cylinder needs a 4mS injector "on" time to maintain 14.7:1 AF ratios. In a sequential system, the PCM will open each injector for 4.5mS at the appropriate time. In a batch fire, you simply divide that by 4. So the same setup in a batch fire system will fire EVERY injector for 1mS per revolution instead of firing EACH injector for 4mS. That way, in a batch fire system, by the time said cylinder reaches it's intake stroke, it's holding 3/4 the amount of fuel needed behind the intake valve and the last 4th needed is received when the intake valve opens.
The difference comes from efficiency. Liquid fuel flowing in from a closed intake valve doesn't burn nearly as well as a single, atomized charge directly from the injector. So yes, batch fire systems are "less efficient" but not by a gigantic margin.