No you can't do it with the same investment of time. 4 retainers use twice the number of ventures as 2 retainers, so you will need to obtain the ventures. You'll double your time investment to get the ventures for that, so it's hardly the same time and effort is it?
I never said you needed to farm ventures. The point I was making is that the player can chose to do something that earns ventures in order to fuel their retainers actions, or they can chose to gather/farm whatever items themselves, or craft, or do something else entirely. It's the player's decision how they spend their time, and if they wish to play content that rewards them with ventures, then they can use their retainers. If not, they can't. It's not like the retainers automatically generat gil or can obtain ventures for each other.
I don't think you get it. The amount of gil a player has depends on the player's actions. Retainers do not automatically generate income for the player, the player must make use of them to do so. Acquiring gil is a consequence of multiple activities within the game, and not specific to the use of a retainer to sell items on the market board. The retainer itself does nothing but stand there when called, the player has to actually do something to use the retainer as a tool to earn gil.
It seems like you and others want to use the fact that 4 retainers lets you put more things up on the market board at once, or that through ventures you can use 4 retainers to gather 4 times the items - which in turn leads to more income. The thing is, you have to have things to sell on the market board to use your 4 retainers, and you have to earn the ventures needed to send retainers out gathering/farming. It's entirely dependent on the effort that the player puts into it.
Yes, having more retainers will reduce the time taken to earn gil selling items - assuming that you have the items to sell. Since you have double the number of retainers you need double the number of items to sell to realize the larger income you say will be generated. Those items do not come from no where, they come as a consequence of what the player does. The retainers do not offer an inherent advantage in earning gil. You do not earn any more per venture used or item sold, it alters nothing about that at all.
You say that's all game play, fine it's all game play, but does me having 4 retainers mean I can sell things for twice the price of someone else? Does it mean that I get a discount on the number of ventures needed for sending my retainers out? Of course it doesn't, so where is the advantage?