I'm no mixer, but
Part 1: It seemed like a good intro, though I do agree that it could be a bit longer to make a nice background music loop.
Part 2: I'll agree with you that the mixing could be better, it seems kinda awkward, as it is.
Part 3: This part is pretty good, but I think that towards the beginning the sound of some object hitting a wire (very professional description, I know) clashes a bit with the background, but once more instruments are added in later, it evens it out a bit.
Part 4: My favorite one, reminds me of some of the major plot line scenes in Golden Sun, which is my favorite game duo (hopefully trio soon), so it makes me very happy.
Overall: I think that with a little work, these pieces could be made into amazing soundtracks, and you yourself pointed out almost all of the flaws (The rest can be seen in reviews). But, even as they are, these would be perfectly acceptable in video game settings.
1: 9/10
2: 8/10
3: 9/10
4: 10/10
O: 9/10