wget is THE standard Linux tool to download files via http(s) or ftp on console and of course Gentoo's emerge is using wget as well.

But wget prefers IPv4 over IPv6, meaning if your download mirror supports IPv6, wget doesn't use it by default. You can change this behavior by setting

prefer-family = IPv6

either in /etc/wgetrc for everybody or in $HOME/.wgetrc for a user only.

So enable it and tell portage to use an IPv6 enabled mirror, by setting GENTOO_MIRROR in /etc/make.conf. You'll find a list of IPv6 capable mirrors on gentoo.org (look for mirrors marked with *).