With which "non-trump" holdings is a 4-4 fit superior to a 5-4 fit?
This is sort of an odd question. The cards you get are the cards you get, and you should do the best you can with the cards you have. You don't get to decide whether you have a 4-4 fit or a 5-4 fit, so in a sense this information doesn't help you. But as a way to improve your hand evaluation, let's consider this.

One answer comes from the law of total tricks, which basically says, if you add up the tricks your side can get when declaring in your best trump suit to the tricks your opponents can get when declaring in their best trumps suit, it'll add up to the number of trumps you have plus the number of trumps they have. Of course, this isn't actually a law (it's easy to construct counterexamples), but it's a good rule of thumb. In this case, you're comparing 8+7=15 to 9+8=17, so it seems likely that your 9-card trump fit will have more trick-taking potential for the same number of high card points.

You can also look at this from the losing-trick-count point of view. If you move a spot in a 2- or 3-card suit into a suit that already has 4 cards, you're removing a loser from your hand (in losing trick count, you count the number of cards that will lose to an A, K, or Q not in your hand, so AKx, AKxx, and AKxxx all have one loser, while xxx has 3 and xx only 2).

Even good old high-card-point evaluation methods typically give some sort of value to distribution -- either adding a point for the fifth card in a suit, or adding points for having doubletons instead of tripletons, singletons instead of doubletons, etc.

So this is a lot of ways to say, your intuition is correct. All things being equal, in a pair of hands that only have one fitting suit, having more cards in that suit tends to increase the number of tricks you can take.