How we cite our quotes: (Book.Chapter)
Quote #4
Sure, life is a small thing, and small the cranny of the earth in which we live it: small too even the longest fame thereafter, which is itself subject to a succession of little men who will quickly die, and have no knowledge even of themselves, let alone those long dead. (3.10)
Marcus just loves to talk about how tiny we are. He insists that he keep this smallness of existence constantly in mind so that he doesn't ever value his individual life too much or place too much value on transient things like fame or reputation. This is a kind of reality check that's meant to keep the emperor focused on the things that matter, like doing social acts to benefit the common good.
Quote #5
Look at the speed of universal oblivion, the gulf of immeasurable time both before and after, the vacuity of applause, the indiscriminate fickleness of your apparent supporters, the tiny room in which all this is confined. The whole earth is a mere point in space: what a minute cranny within this is your own habitations... (4.3.3)
Marcus telescopes out to see the earth and human life as it really is: super tiny, completely insignificant. He doesn't do this to depress himself, though it seems like this would fit the bill. He wants to keep perspective on himself and the world he lives in. Remember that he's always addressing himself in these chapters, so when he exhorts (i.e. "Look...") he's telling himself what he has to do to maintain his philosophical principles.
Quote #6
You have subsisted as a part of the Whole. You will vanish into that which gave you birth: or rather you will be changed, taken up into the generative principle of the universe. (4.14)
Marcus speaks of the dissolution of death in terms of a great recycling program. It isn't as though he will be going somewhere foreign or scary, but he will be heading for change. As soon as those component parts are broken up, the Whole will figure out what he'll be next. For Marcus, there are two basic principles of life: life is change, and the individual life is part of a larger system. While dissolution sounds like a bad deal for the individual, it's really a mere rendering back to the universe what it let you borrow for a while.