The four virtues, as Marcus notes here are:
- truth (wisdom)
- self-control (temperance)
Practicing and living these virtues is how you find contentment and happiness.
Furthermore, practicing the three Stoic disciplines, where are:
- self-sufficiency of your mind (discipline of assent)
- act in accord with true reason (discipline of action)
- accept your inheritance of fate (discipline of desire)
will bring you contentment and happiness.
Marcus boldly tells himself, if he can find anything better than these virtues and disciplines, he should turn to it with all his heart. But if nothing is better than the god within, which has controlled his impulses and which has helped him scrutinize (examine or inspect closely and thoroughly) his thoughts and has helped him "take care of men", then he should not give any room for other philosophies.
He says it isn't right that rational and social good should be "rivaled" by any different order - such as vanity, power, prestige, money and pleasure. All these pursuits may bring short-term happiness and joy, but ultimately they control you and carry you away to a place that ends in dejection and a life of emptiness. (This is why Stoicism wins over Hedonism).
Arriving at this conclusion, what is left? To "simply" and "freely choose the better and hold to it."
(See also Citadel pp. 124, 179, 186, 237-238, 242, 265)