I've been doing some research on this today and apparently, it is legitimate. First, I found the game listed on Amazon Japan, where Google translates the title as "Phantasy Star - Sega Game Honpo - The End of the Millennium." Unfortunately it says they won't ship outside of Japan, or I'd try to buy this with some Amazon balance I have.
I did some searching on "Sega Game Honpo," which was an unfamiliar term to me. It was a Japan-only Sega game download service running from 2003 to 2006. Sega used it to sell cheap PC versions of Mega Drive and Mega CD games. A Japanese Wikipedia page lists PSII, III, IV, and the eight PSII Text Adventures as some of the games on this service.
Amazon (Google translated) lists the game as being made by "Mediakaito." The Wiki page for that company lists Sega Game Honpo as one of their projects. Also, the Sega Game Honpo Wiki page says "Mediakaito" sold packaged PC versions of some games, which would be where this version of PSIV comes in, I suppose.
The relevant links:
Phantasy Star - Sega Game Honpo - The End of the Millennium on Amazon1up.com announces Sega Game Honpo opening (article date is apparently incorrect)
1up.com announces new games on Sega Game Honpo (article date is apparently incorrect)
Mediakaito on Japan Wikipedia --
Google translate versionSega Game Honpo on Japan Wikipedia --
Google translate version"Mediakaito" official site"Mediakaito" official site - Sega Game Honpo support page (Google translate version)