MtG Arena: Fifth card after draft or selaed

Solution 1:

For the moment, 5th copies of cards contribute points to something called The Vault, which requires 900 points to open.

Commons are worth 1 point, Uncommons 3, Rares 5, and Mythics 10.

When you open The Vault, you receive one Mythic, two Rare, and three Uncommon wildcards.

The Arena Devs have stated that they are seeking new solutions to the "5'th copy problem", so this is likely to change before the game leaves Open Beta.

Source

Solution 2:

As of January 17th 2019 5th Card protection has been modified fairly heavily. However the changes as they apply to limited formats (Sealed, Draft, etc.) are fairly minor. You will still see in your card pools cards you have 4 copies of, and be able to choose them as normal. Going forward, when you receive a fifth copy of a:

• Mythic, you will receive 40 Gems
• Rare, you will receive 20 Gems
• Uncommon, you will receive 3 points of Vault Progress
• Common, you will receive 1 point of Vault Progress

All of this will be applied when you open your packs in sealed, and after you finish picking your cards in draft¹. The vault otherwise remains unchanged, it is still 900 points total to open and you will still receive 1 Mythic, 2 Rare, and 3 Uncommon wildcards. However since you no longer receive progress from duplicate mythics and rares, it will be slower to complete.

^{1: This timing makes things kind of interesting, as you technically have the ability to go craft a play-set of a particular rare or mythic after you drafted a copy but before you finish picking all your cards. This could be a (really bad) way to get a couple gems if you are desperate for some reason.}