Hey all 
This topic will cover the confirmed and suspected enemies (and factions) within Halo 4, as I have seen many questions regarding this. I plan on keeping this updated as new information is released, bringing as much information about the enemies as possible. All input would be very much appreciated, as I won’t be able to find everything, and (although I have read both Cryptum and Primordium) I’m no master of Halo lore! 
Anything not mentioned in the summaries is either not-confirmed, or I don’t know about it. If you’re not sure, or have extra information, shoot up a post, I’ll be interested to read it! Also, If you think you can re-word my (often mishappen :P) sentences, or just have any additions, feel free to tell me.
Quick summary of Known Factions and Enemies
>Forerunner/Promethean
–>Crawler
–>Knight
---->Normal/Default/?
---->Lancer
–>Watcher
→ Ball/Sphere (seen in campaign trailer) [Revealed to be a Cryptum, by the press release]
>Covenant
–>Grunts
–>Jackals
–>Elites
Quick summary of Suspected/Theorised Factions and Enemies
>Forerunner/Promethean
–>The Didact/Forerunners
–>Sentinel [Regular/Enforcer/Super/Enforcer]
–>Engineers [Huragok]
>Covenant
–>Drones
–>Brutes
–>Hunters
–>Prophets
>Other
–>Flood
In-depth Summaries [Speculation will be in italics]
>Prometheans: The Prometheans are the “highest form” of warrior-forerunner. It therefore makes sence that the combat-AIs seen in the previews are Promethean built. Note that these AIs [Crawlers, Knights, Watchers] are not actual Promethean Forerunners. It is currently unknown whether all Forerunners, if there are even any left, are on the same faction.
→ Crawlers: Dog-like Promethean combat AI. Seems fairly weak, doesn’t appear to have weapons. However, being very fast and having a reasonable melee attack, they can apparently overpower through numbers. Numerous crawlers can be summoned by a Watcher.
→ Knights: Humanoid Promethean combat AI. Has been shown to weild both Scattershot and Light Rifle. It is likely that multiple types of knight exist, as hinted toward in the Gamespot interview. “The Lancer” has been revealed to have the ability to teleport short distances. It is unknown whether it is the Lancer, or another type of Knight, that can spawn up Watchers from their back.
→ Watchers: Flying Promethean combat AI. Are able to attack with some kind of in-built gun. Have the following abilities; creating a temporary shield; spawning crawlers; “throwing” back thrown grenades.
→ Cryptum: The gigantic ball that Master chief encounters during the footage is a Cryptum. A cryptum is effectively a “Stasis chamber” that allows (to our knowledge, only forerunners) to be preserved, possibly indefinately. In the novel, a Forerunner’s presence was required to open the Cryptum. (Thanks to raindoc for pointing out the press release photo name confirms the sphere as a Cryptum.)The appearance of the cryptum hints toward a Forerunner’s return. And, if there’s one cryptum, there may well be more.
>Covenant: Cortana refers to a group of two Grunts, a Jackal and an Elite as “covenant” in the gameplay demo. Although we can’t be positive, we can assume that these covanent forces are still followers of “The Great Journey”, or they are outcasts from the (assumed) disbanded covanent forces. If these theories are correct, we can expect more than Grunts, Jackals and Elites. However, nothing has been confirmed.
→ Grunts: Seem like the generic grunt characters from the previous Halo’s, with a nice graphical update.
→ Jackals: Again, generic Jackals from the previous Halo’s. It is unknown whether rifle-weilding Jackals will be in Halo 4.
→ Elites: Again, generic Elites from the previous Halo’s. Only Minor,Major and Zealots have been revealed thus far.
Suspected or Theorised Factions or Enemies
The following discussion is speculation, either due to hints from novel/trailers, or logical progression from earlier games. If you have a valid theory, call it out here; I might make a list of theories in this section.
>Forerunners/Prometheans: It is unknown whether actual Forerunners will be in Halo 4. So far, we have only seen the involvement of Promethean combat AI. This has led to speculation that Forerunners, specifically the Didact or the Librarian, will be in Halo 4. Remember that Forerunner machines/AI have been in previous Halo’s, without the Foreruners (Sentinels, from Halos 1-3). The involvement of the Cryptum also further indicates Forerunner presence.
→ Sentinels: Sentinels have played an important part in the Forerunner background of Halo (both in novels and games), and are speculated to be in the game. No evidence except this has been revealed.
→ Engineers: Engineers are Forerunner creations. They are therefore speculated to be in Halo 4. No other evidence supports this. They are known to be non-combatents (except when made suicide bombers, or shield generators by covenant).
>Covenant species: As Cortana refers to the Elite, Jackal and Grunts in the gameplay footage as “Covenant”, it is speculated that Drones, Brutes, Hunters and Prophets will be in Halo 4. No other evidence supports this.
>The Flood: It has been confirmed that we will not be fighting the flood in Halo 4. This does not cut them out of the picture completely; they may be there to help us, or even just for canon reasons.
Link/Source dump:
Enfinit’s News and Link coverage: Halo - Official Site (en)
Greenskull’s campaign analysis: http://www.youtube.com/watch?v=ZvNwq4lYBrc&feature=plcp
Ducain’s Spartan Ops gameplay analysis: http://www.youtube.com/watch?v=946IzigqX5g&feature=plcp
Gamespot’s 343 gameplay interview: http://www.youtube.com/watch?v=00_EyaZpbsU&feature=plcp
Xbox press release (thanks raindoc): http://press.xbox360.com/products/p51/halo-4
Again, if you have suggestions, improvements,additions, comments, or just want to speculate or discuss content, leave some posts ^_^. I hope you had a good read.