As others have stated, don't do it... ever. Also keep in mind that 4-battery mods typically use a parallel/series battery connection scheme. Different batteries have different voltage sag, which is the resting voltage of the battery minus the voltage that the battery will supply whilst you put it under load (which also implies the voltage sag is dependent of the load, measured in amps). In addition, the resting voltage goes down after every puff because each puff uses up some of the charge left in the batteries. You can measure the resting voltage of a battery after letting the battery rest for a certain period of time. (After about 30 seconds will get you reasonably accurate readings.) The rate at which it goes down is dependent of the battery make and model, the load you put on the battery and for how long a period of time (and how the load changes during that period), the amount of charge left in the battery, age of the battery in conjunction with the amount of wear and tear the battery has been accumulating over time, temperature of the battery, and also variations that are due to the manufacturing process of the battery. (That is, assuming the battery hasn't been damaged of course.) Now, because the voltage sag also changes depending on so many factors... there's always going to be a certain imbalance between your four batteries when you put them under load by puffing. But with one set of two batteries hooked up in parallel, or with two of these sets hooked up in series (to give you 4-battery parallel/series), what happens is the weakest battery in each set receives the highest load. This imbalance, then, impacts the rate at which each battery in the set will be discharged. I.e., as the batteries continue to be discharged on every puff, the imbalance also grows worse with every puff, and, in addition, the rate at which it continues to grow worse is accelerating on every puff. The good news is these 4-battery regulated mods will normally throw an error message if the imbalance has gotten too big to be safe. The bad news is the weakest battery might start to vent long before that happens, if you didn't teach yourself ohm's law and apply correct rules of battery safety. This and the fact the resistances of the contacts between the batteries in your mod also are imbalanced to some degree is why with 2 batteries in parallel you don't just double your CDR of your battery, but instead you take 15 percent off after you double it. Take off more and you get more safety still. If you stay within 20 - 30 percent above the CDR rated by Mooch you'll be relatively safe as long as you keep in mind that there's always a risk because Li-ion rechargeable batteries can start to vent regardless of the load on the battery as well as regardless of the battery temperature. I don't always blow my face off. (But when I do, I do it safely.)