It seems the developers have confirmed the time span of the new game as a little “broader” than Age 2 (maybe up to late Renaissance?). So how shall gunpowder be depicted in the late game?
Well, though Age 2 campaigns do cover the time period as late as Battle of Lepanto (1571), the technological development of the main game seem to end up in late medieval. For example, the most advanced firearm that most civilization can have access to is the hand cannon, and that’s mainly a 14th or 15th century thing, and mostly fell out of use at the time of, say, Lepanto:
If Age 4 is advancing to a little later time period than Age 2, then maybe more advanced firearms like arquebuses should be available (though there’s a technology called that in Age 2). They had been widely equipped since 15th century and were known for the use by Janissaries.
Also, Renaissance artillery were already well-developed, with portable, fast-firing and accurate artillery pieces like culverins that can be deployed in field battles, not just the “lightweight trebuchet” kind of thing as in Age 2.
I don’t really think the game will go further into the era of flintlock muskets and line infantry (that’s a totally different kind of warfare, though would also be fun to play). However, I really expect to be able to field a broader range of more advanced firearms and artillery pieces in the late game than what we have in Age 2.