If a partnership has both a 4-4 and 5-4 holding on one deal, I could see why it would make sense to have the 4-4 holding be the trump suit. The 5-4 holding can take "long" tricks with the fourth and fifth cards (on the 5 side), while the only way to "lengthen" the trump holding is through ruffs.
But suppose that you have the 4-4 and 5-4 holdings in separate deals. And suppose that you have no other "long" (seven card or longer) suits in either deal, so that the eight card (4-4) holding is distributed 8-6-6-6 in the four suits, and the nine card (5-4) holding is distributed 9-6-6-5 in the four suits.
In this case, wouldn't it be better to have the 5-4 holding with the extra trump than the 4-4 holding? Especially since the ninth card guarantees a ruffable imbalance, e. g. 5-3-3-2 opposite 4-3-3-3, whereas the eight card holding could be 4-3-3-3 opposite 4-3-3-3? Or is there something I am missing?