A typical drip coffee contains roughly 100mg of caffeine per 8 oz cup, which means a 30 oz cup of coffee would contain very similar amounts to one of the charged lemonades in question. Or course, caffeine varies wildly in coffee depending on exactly how it's brewed as well as bean origin and roast, so you could easily see well over 400mg in a 30oz drink. And let's not even get started on adding extra shots of espresso.
A few corrections:
400mg of caffeine is not considered a daily recommended maximum, but "an amount not generally associated with dangerous, negative effects" (FDA). Most people can consume more than that and have no significant side effects.
The Charged Lemonade has more caffeine than most any other drinks... When you fill up a 30 oz container with it and don't add any ice. If you filled up the same container with an energy drink or coffee, it would have similar amounts of caffeine.
Aside from the possibility to associate "charged" with electrolytes, none of the marketing or branding of the drink implies that it's an electrolyte drink. Personally I would much sooner think that charged means energy, i.e. caffeine and the like, but that's just me.
A healthy adult consuming multiple of these beverages wouldn't likely experience any heart issues, but likely would have things like jitteriness and have a hard time falling asleep. And while people with heart conditions should definitely stay away from these drinks, "would go into cardiac arrest after one" is incredibly overblown. If that were the case, the deaths would be in the hundreds or thousands, not one or two.
Panera has only "reduced" the caffeine amounts in the drink by adding ice into the cup because they are now behind the counter, not by changing the formula of the drink.