I get what you're saying, but the question asks for the minimum number of weighings required to be CERTAIN which marble is the heavier one. So if you split it into 4 per pan, only one of the possible paths requires one weighing, the other pathway requires 3 weighings, so you can only be CERTAIN which marble is heavier after 3 weighings.
If the 4 marbles in each of the two pans aren't of equal weight, then you can't be certain which marble is heavier after just one weighing, right?
So like, from one action: placing 4 marbles in each pan, you get two possible outcomes. The scales balance, or they don't. If they balance, you've worked it out in one weighing, if they don't balance, you need to do 3 weighings.
But the idea is that from just this one action, you cannot be 100% certain the scales will balance, so you cannot be 100% certain that it will only take one weighing to find the heavier marble.
Does this kinda make sense?