Stanza 4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

BODY
But physic yet could never reach
The maladies thou me dost teach;

The body wrestles the mic back—after all, it's got arms, right?—and rebuts the soul on the same subject: sickness. But something's changed. The body has turned on the soul directly. Just look at that accusing "thou" planted in the middle of line 32 like a punch in the chops.
And the body doesn't stop there. After kicking up the tone with some second-person aggression, it ratchets up the subject to a whole new level. The soul may carp about feeling sick and then getting well again, but the body endures pains that no medicine can even touch.

Whom first the cramp of hope does tear,
And then the palsy shakes of fear;

The soul introduces the body to a range of incurable emotional pain which the body catalogs in this stanza, using metaphors of physical pain. That's no surprise there since it's a body after all. Use what you know, folks.
Although they're grouped in pairs of "good" and "bad" feelings, this stuff is across-the-board bad to live with.
First off we've got hope, compared to a cramp, and fear, which is like the uncontrollable shaking of a nervous system disorder.
Notice how the metaphor fits the feeling: hope nestles inside you like a cramping muscle while fear affects your whole body, literally making you shake.

The pestilence of love does heat,
Or hatred's hidden ulcer eat;
Joy's cheerful madness does perplex,
Or sorrow's other madness vex;

In the same way, it makes sense that love is a pestilence, some kind of aggressively infectious disease like the plague. Anyone who's ever had a Bella-on-Edward-style crush will know that love is definitely a full-body condition, delivering heat and plenty of it.
Hatred, on the other hand, is a hidden illness, nursed in secret like a stomach ulcer. It gnaws at us in the same way.
And check out the attention-getting alliteration of "hatred's hidden." More on that in "Sound Check."
The final positive-negative pair is joy and sorrow. Both are compared to "madness," which for Marvell meant "craziness" or "mental illness," not anger.
Joy's actually not so bad, although the mental giddiness it produces can be confusing and distracting. That's why happy people tend to be goggle-eyed ditzes. (No offense, all you smilies out there.)
Sorrow's madness is at the other end of the spectrum: destructive and consuming, it drives us into depression.

Which knowledge forces me to know,
And memory will not forego.

All this knowledge—joy and sorrow, fear and hope—is forced on the body by the soul. And that isn't all. Not only does the body have to feel these things in the moment they happen. It also has to remember them.
By encoding them in memories, the soul ensures that the body will remember sorrow and fear, even when the bad guys are long gone or a new pet cat has replaced the run-away. Crafty, right?

What but a soul could have the wit
To build me up for sin so fit?

Lines 41-44 mark the body's extra speech time. This match has officially gone into overtime, ladies and gentlemen. The body's got the ball, and it's kicking things up one last time.
Everyone's been cranky in this poem, but here the body offers up a more somber, way more serious complaint than anything that's gone before: nothing but a soul would be clever and malicious enough to make the body commit sin.
So how did we get from "joy's cheerful madness" to "the soul made me kill that man"? Well, by giving the body consciousness, emotion, and a sense of morality, the soul groomed the body to make mistakes and fall into wickedness. If you're being eaten alive by hatred's "hidden ulcer," then it's not such a stretch to start doing bad things to the people you hate.
Sad but true.

So architects do square and hew
Green trees that in the forest grew.

The body illustrates its final point with an interesting metaphor. Architects cut down trees and carve them into more civilized shapes, like chairs and sheds and stairs and beds (sometimes the architects are poets too).
In the same way, the soul captures the body in its state of natural ignorance and makes it human.
Being human comes with a lot of mental baggage: we're talking emotions, morality, and the consciousness that you exist in a vast universe. All that baggage means that humans are capable of feeling hurt and getting angry, of knowing what is evil and then doing it anyway, and then being tortured with religious convictions that human sin will be punished. No wonder the body just wants to be a body!
In a final sonic flare, the assonant E's of "green trees" emphasize the importance of the body's last argument.