Question about 4 Sticks of Ram

Mixing RAM is a complicated mess. You shouldn't do it. The reason is that RAM modules have more specs to them than just those commonly referred to as "The Big Three". But, for the sake of discussion, we'll go ahead and assume that there aren't any complications if you install the additional RAM modules. Using your example, yes, if you use those sticks in the unoccupied slots, you'll have dual channel.

Now, for your query about using more than 4GB... It depends on what those "multiple applications" are that are running. If you're doing music composition, video rendering, etc., those require a hefty amount of RAM, too, and you should consider using more. Another consideration to take is if you're using a discrete graphics card. If you are, then the GPU has its own RAM supply and won't touch the RAM you've installed. If you're using an integrated video chipset, you'll probably want to increase your RAM capacity.

On the other hand, if you're just doing simple tasks like web browsing, or listening to music on say, iTunes, then you're fine with 4GB.