> I think most Perl4 programs will run on the latest Perl 5.

I think most programs that will run on the latest Perl 5 will run on the latest Perl 6.[1]

So, yeah, feel free to roll with Perl4 and maybe mix it up with a little Perl 6 magic (eg concurrency or Unicode grapheme string handling).

[1] https://github.com/niner/Inline-Perl5/