4 - Lightning Dust deserved way better. She could have been a fantastic rival character.
That one doesn’t seem to be all that unpopular, particularly considering that:
a. When her tornado destroyed the balloon containing Rainbow Dash’s friends, it had entered Wonderbolt airspace without authorization - they effectively placed themselves in danger.
b. Spitfire openly condoned Lightning Dust’s reckless behavior right up until Rainbow explained what was going on, which would already have been obvious if the cadets had been meaningfully supervised during training.
And for what it’s worth, “The Washouts” had her offer Scootaloo what could legitimately have been a quality alternative to the Wonderbolts. Letting her learn that safety rules aren’t all bad could have yielded a lovely lesson in finding something noteworthy to do with one’s life despite a disability.