the prometheus firmwares often called v1, v2, v3, and v4 are actually not numbered.
there never was a prometheus version released as v3 or v4, news/release sites just added that to keep track of the different releases themselves.
the new v2 is actually the first time the creator of the firmwares added a version number.
EDIT: Razor1993:GEN D3 is NOT recommended.
it's better to update from D2, because using D3 may cause corruption with some game saves.