So you continued outright ignore dev team and director inteviews made after the games development finished(october 2007, four months before release was the date of Kobayashi's ten year time gap between dmc3 and dmc4, after development had essentially finished), and confirmed timeline of one year between dmc1 and 4 as the anime was the events between them, again confirmed by Kobayashi, who wrote some episodes of the anime, thus CEMENTING it's canonical status...
So yeah, DMC4 is one year after DMC1, and Dante was 29, not nearing forty. While Agnus may have had Yamato for some time(how the hell he even got it is a mystery since Nero Angelo's Zweihander is NOT Yamato, and this Yamato would probably have been left somewhere in the Demon World after Vergil was killed and soulforged into Nero Angelo), the fragments of Vergil's soul would not have been for more than a year at best...how they even GOT pieces of the soulforged armor brings up MANY questions... You may have your pwn personal canon, but the offical canon says Mallet Island and the resealing of Mundus in DMC1 was one year prior to DMC4 occuring.