There should be an ebuild for the IPTables::IPv4 module in portage, if it is possible. Reproducible: Always Steps to Reproduce:
Unless this module fills a dependency for a package in portage, then please use the g-cpan.pl script to install it instead. # g-cpan.pl IPTables::IPv4
Please reopen this bug if you can identify a package that should be depending on this and isn't. Otherwise, we can't add it to portage (see policy guide).