Why I can't buy Terraria 4-pack as gift???

You can't (normally) buy 4 packs as gifts. This is because the 4 pack adds 1 to your library and 3 to your inventory.

There is a work-around, but I don't know it.

Mighty Gabe decided you shall not buy it :P
pleasantries aside I have no idea really, try to reopen the client. If its still not working, contact the support. Does it shows anything as an error message?

Because it says "Give the extra copy to a friend!". As in, you get your own copy. Not "Give all the copies to friends!"

because gifting a gift that gifts gifts would be too much

Well maybe because you already have it, and the deal is pay for 3 copies and 4th copy is for you and since you have a copy linked with your account you're not eligable for another 4-pack

One copy is free under the assumption that you yourself will receive one, therefore this happens automatically (and is disabled if you already own it).