Sorry, I was kinda being a bit sarcastic with that sentence. While, yes, 5* Servants are usually used because of their potency, it does not make 4* any less decent if it has decent performance on it's own.
Otherwise, I'm expecting a 'Euryale and Robin are garbage' from you as well just because they somehow cannot compete against most 5* ST Archer Servants outside of damage. They still perform. And they still perform well enough to be really good Servants because they can output damage while being in that rarity bracket. Both have been impacted by bosses with permanent debuff denial and/or debuff removal on Break. I still call them top-tier because the enemy suffers hard if they get to work their magic. Because they're evaluated based on their rarity as well.
Same case applies to Idol Alter. For her rarity, she is decent. It's hardpressed to call her good because she needs double Summer Skadi + Oberon for looping but at least she's decent at her job.