Ah, talking about Mugen Senshi Valis, are we? Yeah, let me explain. All of them originated on the MSX (except IV & X, but more on that later). The 5th, called Valis X was made by a different company, Eants, who bought the series' right from the original owner, Telenet Japan.
Valis: we got it for Genesis (all systems listed here are US releases)
Valis II: TurboGrafx-CD & then another port called Syd of Valid for Genesis
Valis III: 1st Genesis, then TG-CD a year later
Valis IV: SNES (yeah, 1st Valis game NOT on Genesis or TG-CD)
Valis X: As explained above, series' rights got sold to another company (Eants) who makes hentai games. It's actually called Valis CROSS, not 10. It's also a 5 parter series.
Here's a good page to read