The amount of tables, you play at the same time, does not impact variance and therefore also not the required bankroll. It might have a small impact on your winrate, but so many other factors impact winrate, that its negligible. At least if we are talking something reasonable like 4 tables.
However 2/4 is a very high limit online, and the fact, you are asking such a basic question, makes me think, your skill set is probably not ready to compete at this level? The whole idea of a bankroll is, that you will never go broke due to variance. But if you are a losing player, you will eventually go broke. Its only a question of time.
Even if you have 40k set aside for poker, if you are new to online poker - maybe from a live background? - you should start in the micros and only move up, when you beat a certain limit over at least 20k hands. Maybe you dont have to start at 2NL, since you already know how to play poker, but 400NL is way to high. Even a 10NL online game is significantly tougher than your typical 1/2 live game in a casino.