The 4 seiyuu are:
* Tsubasa Yonaga (代永翼) - Voicing for Haru Irei, the protagonist.
* Nobuhiko Okamoto (岡本信彦) - Voicing for Kenta Sakazaki, the protagonist's friend.
* Mariya Isei (伊瀬茉莉也) - Voicing for Rina Noyama, the girl.
* Hiroshi Kamiya (神谷浩史) - Voicing for Kowaji Matsumoto (?), one of the villains (?)
Hiroshi Kamiya is the seiyuu for Tieria Arde from "Gundam Double O" for those who don't know this already.
From left to right: Hiroshi Kamiya, Mariya Isei, Tsubasa Yonaga and Nobuhiko Okamoto.
Images of the characters:
Haru Irei, Rina Noyama
Kenta Sakazaki, shop owner
Kowaji Matsumoto (?), Boris Shawer (?)
"Gunpla Senshi Gunpla Builders Beginning G" Part A can be watched at Gundam Super Expo 2010, currently ongoing at Tokyo Dome City Prism Hall till August 19th. It will also be shown on Japanese TV, on August 15th at 7.30PM through Channel BS11.
The promo video of "Gunpla Senshi Gunpla Builders Beginning G" Part A is up on the official site as well, but I can't seem to be able to play the movie.
Images are from Gunpla 30th Anniversary's official site: Link 1, Link 2.