You said the Laptop has TWO RAM slots? and is it preloaded with ONE stick of 4GB?.. if so, then you are right, but... for best performance you should not mix sticks of different size or speed... So, you either add one identical stick to the already installed (same brand, size and speed and if possible model), OR you buy two new sticks of the same brand, size, speed.
And if your budget doesn't allow buying two new sticks, you could go ahead and buy what you can, but at least make it of the same speed, there is no getting around that if you want to avoid lockups and different types of performance issues. Different brands may have different technology and work different so that may be an inconvenience.. size may be the least important characteristic, but if you need to get the best performance out of it, you have to install identical sticks.
EDIT: power required by RAM modules is another important characteristic, different brands and even models can require a small difference therefore the need for them to be as closely matched as possible.
Battery life or charge will not be affected if anything it should save battery, as more ram can get things done faster and save time, therefore less use and less frequent recharges which may ultimately determine battery life.