I sincerely doubt theres going to be a new XP. they already released Civ4 complete, and judging by the amount of time between previous editions of civ, the time is about right for a new one in about a year. also, they have never released more then 2 XPs for an edition.
Civ 4: Colonization is as close to an expansion package as we are likely to get, but I don't know whether one can infer from its release, with enhanced graphics but a similar engine, that there are going to be more games based around the engine first before moving on to the next engine for Civ 5. I would like to see Civ V as soon as possible but first of all I'd like to play all the possible mods for this one.
I suspect that no more XPs will be released for Civ IV as BtS has taken the game to bursting point and Colonization uses the engine to do a slightly different thing. I think Civ 5 would introduce (or reintroduce - e.g. animated advisors) concepts missing from Civ IV plus a new graphic design (I'd like to see random events taken to a level such that they are no longer random events and are an integral, seamless part of the game like espionage became in BtS, but I think that would require an overhaul of the engine to make the terrain more fluid: not merely e.g. anthropogenic global warming coming at the end of the game but climate change as a fluctuating part of the natural environment: we have been through the Little Ice Age, thriving pastoral colonies in Greenland have become uninhabitable tundra, viticulture also was an integral part of the economy of southern England at the time of the Roman occupation; cities were abandoned, became free states, died and were resettled on a much larger and more changeable basis than Civ has previously allowed for - e.g. Danzig, for instance; civil war is also something that has plagued more nations and is not adequately represented by periods of anarchy...). Patching these into Civ IV would be difficult even in an expansion but could all be done for Civ V.
Given that the Civ IV engine is still usable and presentable I think that it would thus take longer for Civ V to come out, just like the Baldur's Gate engine really precluded Black Isle from developing another RPG system until they really couldn't get away with it any longer and made the switch to 3D after D&D Third Edition came out and made their gameplay engine all but obsolete (Icewind Dale II, done under Third Edition rules, was a nightmare to play compared with its predecessors because Third Edition significantly altered combat and character creation such that the game was more fluid than Second Edition and therefore lost the familiarity the BG engine had). Civ IV's engine is such that it could be used for a lot of different sub-species of the TBS game, and it is such a modder's paradise that Sid might want to wait until the mod tide ebbs to see what he wants to put (or put back) in Civ V.
I think there will be one, but Sid has only just finished with Civ Revolutions and Civ IV: Colonization, and would as likely as not want to keep speculation about Civ V to a minimum until after both games are out and have established themselves properly in the market. Civ IV strikes me as more versatile than Civs III or II, therefore the length of time between Civs could be lengthening as the game becomes more open to third party mods for it.
@sporr - Welcome to the forums. I agree, Civ IV to me is such a dynamic game that it is difficult to exhaust its possibilities, particularly for modding (I am trying at the moment to access Rise of Mankind but can't find how to load it from the BtS menu). But I would say that any more expansion packs would need to find a way to really integrate what I've suggested above without changing the fundamental way the game plays, and that now would be trickier to patch in than to create anew. The new patch (3.17) does alter the way the game plays and the difficulty levels of it all, but because they would need to get people to pay for another dozen Civs or leaders or units or whatever, and they have announced Colonization in the mean-time, my hunch is: nothing more except fan-produced mods for Civ IV, Civ V in a while, Colonization and another spin-off or two in the mean time.