I would argue that AoE2, Mythology, and 3, are all fairly different but still following a progression. In age 2 you have a base set of identical units and techs which are shared by everyone, and 1-2 unique units and techs. In mythology, you get a new resource, different ways of acquiring it, two new unit types with new counters, god powers (which are remarkably like aoe3 cards, if often more over the top), and a version of aoe3’s politician system with god choices. And each civilization has very different hero mechanics, costs, ranges of archer units, etc Otherwise it plays fairly similarly to age 2 in a lot of ways, but there’s very large faction differences.
Age 3 games between serious players are frequently over in 15 minutes or less. It has a very fast ramp-up to the fighting stage (you can hit age 2 in under 4 minutes) and losing buildings is a lot more consequential. Defensive buildings are very weak, and really are just places to shelter villagers. You can’t garrison military units anymore, so building an army when you’re under attack can be a challenge. Friendly fire artillery and projectile dodging are gone. Treasures are scattered over the map to fight for and claim, with certain treasures being worth a lot more. Units in proximity to enemy units slow their movement speed, making cavalry really important to catch and kill running enemy armies. Obviously the card and governor system. No trade caravans, but expensive, slow-gathering infinite resource buildings. Natives, mercenaries, trade posts, limited numbers of certain units, powerful card-only units, etc.
There’s already big changes in between the different Age games. I wouldn’t be surprised if there were something similar to a card system in Age 4. Say, a Political Reforms system, where you can make certain choices at various points in the game that increase your bonuses to x, while losing the ability to do y or lessening y’s effectiveness. For a semi-believable example, let’s say that you’re playing as the English, and you have a unique Longbowman unit. Maybe one political reform increases your farming speed at a cost of either making longbowmen more expensive, removing them entirely from being trained, or making them cost more population, or limiting the number buildable. That could easily be in the game, make sense, and be balanceable, while providing valid strategic long-term thinking questions.
You still don’t have to have a card system. You just can. Regardless, a game that offers nothing new to its predecessors just feels like a cash grab. If Age 4 is just re-skinned age 2, there is no compelling reason to buy the new game when Age 2 DE is so new. I wouldn’t necessarily say that a new call of duty or assassin’s creed are bad games, but they’re pretty pointless games, unless you are really just in it to enjoy the story. That’s slightly different, but if you’re using it as a storytelling game, why make a whole new game of Age 4 vs just releasing new single player campaigns for age 2 that cost maybe $2-$5 to buy for those that want it?
There needs to be new/different mechanics to explore/master for the game to have any meaning. It could just be a combination of what worked best in the previous age games. It could be new ideas like manning walls, relocatable bases, being able to make land unusable for farming or spreading diseases, who knows. They should be bold and creative and try to bring new ideas into the frankly stagnant RTS genre, but be willing to say “okay, this isn’t working, let’s dial that back and try something else”.