As an introduction, the Rambam's fourth chapter is devoted almost entirely to health issues. Since, as he stated at the end of his third chapter, the Torah enjoins us to care for our health, the Rambam here elaborates on what he considers a healthy lifestyle. (Keep in mind that Maimonides himself was a physician, highly-regarded in his day (in fact rising to become the personal physician of the Sultan Saladin). He in fact authored a number of works on medicine (in Arabic) -- many of which have been translated in relatively recent times.)
I decided for completeness' sake to translate this chapter in its entirety -- although I offer few or no comments on it. Much of it actually reads as sound advice. However, much of the Rambam's medical advice, though to be sure ahead of its time, is basically medieval. (Though I don't usually take it upon myself to argue on the Rambam, I wouldn't recommend blood-letting today, at least without the appropriate medical supervision (probably psychiatric).) Many of the Rambam's recommendations here are in fact based upon medical advice appearing in the Talmud and earlier Jewish sources.
We are surely instructed to care for our health as well, but in the manner modern medicine considers most beneficial. Thus the Rambam's advice here, in spite of his greatness as a Torah scholar, would have to be relegated to the realm of historical curiosity.
Anyway, with that introduction and disclaimer, I begin the translation:
 Since having a healthy and whole body is integral to Divine service (lit., "is of the way of [G-d's] Name") -- as it is impossible to understand or know anything (some versions add: about the Creator) when one is sick -- one must stay far from things which destroy the body and accustom himself to things which preserve one's health. And they are [as follows]:
One should eat only when he is hungry and drink only when he's thirsty. He also should not delay urination or defecation (lit., 'he should not delay his holes') even one minute. Rather, whenever one needs to urinate or defecate, he should go immediately.
 One should not eat until his stomach is full but should rather eat around a quarter less than his fill. One should not not drink water during his meal, but only a little mixed with wine. And when his food begins to be digested in his innards he should drink that which he needs. And one should not drink excessively even when his food is digested.
One should not eat until he checks himself well that he doesn't need the bathroom.
One should not eat until he has first walked before his meal until his body begins to warm. Alternatively, he should do work or tire himself in some other way. The rule of the matter is that one should exert (lit., 'afflict') his body and tire himself the entire morning until his body begins to warm. He should then relax a little until he recovers and then eat. If he bathes in warm water after his exertion, that is good. He should then wait a little and eat.
 Whenever a person eats he should sit in his place or lean on his left [side].
[Note: In Talmudic times the custom was to eat while reclining on cots. Each participant in a meal typically had his own tray before him. (See Talmud Brachos 46b and Ta'anis 5b.) The Talmud considered it essential that people lean on their left side rather than their right in order to ensure their food would go down the correct passageway. As I once read in Bernard Lewis's _History of the Middle East_, wood was a scarce commodity in the Middle East, lumber trees not generally being indigenous to arid and semi-arid regions. Thus, tables and chairs were an unaffordable and so virtually unknown luxury in the area.]
A person should not walk, ride, exert himself, shake/agitate his body, or stroll until his food is consumed. Anyone who strolls or exerts himself after his meal [risks] bringing upon himself serious illness.
 A day and night together lasts 24 hours. It is sufficient that a person sleep one third of that time, which is eight hours. It should be at the end of the night so that there will be from the beginning of his sleep till sunrise eight hours. One will then be found to rise from his bed before sunrise.
 A person should not sleep on his front (lit., 'face') or his back (lit., 'the back of his neck'), but rather on his side, at the start of the night on his left side and at the end on his right. He should not sleep right after eating, but should wait three or four hours. One should not sleep during the day.
 Foods which loosen the bowels -- such as grapes, figs, berries ('tusim'), pears, melons, the insides of gourds ('kishu'im'), and the insides of cucumbers --one should eat at the start of his meal. He should not mix them with his food, but should wait a bit till they leave his upper stomach and then eat his food. And foods which tighten the bowels -- such as pomegranates, quinces ('prishin'), apples, and 'krustmalin' -- one should eat right after his meal. He should not consume them in too large quantities.
 If a person wants to eat chicken and meat together, he should eat the chicken first. Likewise regarding eggs and chicken -- he should eat the eggs first. Meat of small animals (typically, sheep and goat) and large animals (cattle): he should eat the small animals first. One should always begin with the lighter food and finish with the heavier.
 In the summer a person should eat cold foods, should not spice his food too heavily (lit., 'should not increase [his intake of] spices'), and should consume vinegar. During the rainy season a person should eat hot foods, increase his [intake of] spices, and eat a little mustard and 'chiltis' (translated by Ben Yehuda as St. John's wart). Likewise should one do in cooler and warmer climates, in each place as appropriate.