Doom 4 is now Doom, also is a reboot

[Sacred Jellybean]

Hmm... iterate a series by either a subtitle or year. Tough call. A year is rather clinical, but informative. You can tell when a game came out and where it was technologically, and have a better frame of reference. A subtitle is more interesting and can give a feel of the games theme or story. On the other hand, for sonic or doom that sort of thing is immaterial.