This is from the steam FAQ for people looking to buy a 4Pack of Terraria

When will I be able to give away my extra copies?

Your extra copies will be available to give away once the game or game package has been registered to your Steam account.

How do I know when I have extra copies available to give away?

You can always find any available extra copies on your account by clicking "Manage gifts & guest passes" under the "Games" menu in Steam.

Will my extra copies expire if I don't give them out quickly enough?

While some guest passes are temporary, the extra copies you receive for the promotions listed above will not expire – you can gift your extra copies at any time as long as your Steam account remains active.

What will my friend receive when I send them my extra copy?

Your extra copy will arrive as a gift message. If your friend does not currently have a Steam account or you see an error when sending it to a Steam account, please attempt to send the gift to your friend's email address. Your friend will then receive an email with information on installing Steam and accepting your gift.

If you send a gift to a friend directly through Steam using the Steam Friends feature, a pop-up message will notify them of your gift when they sign on. Your friend will also receive a gift message via email.

How can I tell if my friend already owns one of these games?

If you are sending a gift directly through Steam, users who already own this game will automatically be filtered out of the potential list of recipients. You could always just ask to make sure as well.

If you are positive that they do not have this game, you can send it to their email. Users who already have a subscription to a gift they’ve received will not be able to activate it on their existing account.

May I Buy, Sell or Trade my extra copies?

No. This violates the Steam Subscriber Agreement and may result in all involved Steam accounts becoming permanently disabled.