In the not so brief 8)...
I've been having a little private discussion with some other modders here in the community, and from what I've seen, we could pull into vanilla civ4:
The Great Generals
The new leadership traits
The new civs
The new units
The new buildings
The tech tweaks
If there are new civics (I didn't notice any) those are most likely portable as well.
What we can't do without what I will categorize as "a lot of work" (read as "more work then anyone* is willing to do for free") is the vassalage system. Also, I suspect, without looking, that porting the scenarios themselves is probably a significant amount of work.
Now, even porting all those things, civ4 vanilla won't be warlords, because Warlords adds new modding opportunities and features which are not available in civ4, and at least some of which can not be back ported (again without "a lot of work"). These quotes are from me, not a third party, and so I am solely to blame for poor estimates 8). As my friend Hugh Moore used to quote himself as saying, "Only a real jerk would quote himself... --Hugh Moore". It worked better when he did it 8). And yes, that is his real name. His parents are wack.
(* That I'm aware of)