What to do with extra 4 stars?

Just burn him. You'll get rare prism, which will be useful if you missed some quests...

burn them, its a bit painful, but there will come a day where rare mana prisms are useful.

No reason to feel guilty about burning servants past NP5.
FGO is a game that doesn't allow you to use multiple copies of the same servant in your team lineup (aside from borrowing the same servant from your FP list). Past NP5, burning a servant equal or above 4* does give you rare mana prisms, which is a special mana prism that has a shop dedicated to unlocking past time limited purchases.

Currently, it's not implemented on NA but in the future it will be. So hoarding rare prisms is one other way to get some stuff you might've missed if you didn't play since day 1.

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