In Act 4 of Julius Caesar, what conflict between Brutus and Cassius is revealed?

In 4.3, Brutus and Cassius argue in Brutus's tent, so that
their armies will not perceive discord between the two leaders.  Essentially, Cassius is
angry because Brutus has publicly disgraced Lucius Pella (Brutus claims Pella took
bribes), despite the fact that Cassius had attempted to intercede on Pella's
behalf.

In response, Brutus
accuses Cassius of being greedy himself:

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Let me tell you, Cassius, you
yourself

Are much condemned to have an itching
palm

To sell and mart your offices for
gold

To
undeservers.

Later, during
the same argument, Cassius claims that he is "older in practice, abler than yourself /
To make conditions." Brutus, offended by this claim, misinterprets this statement and
thinks that Cassius has claimed to be a better soldier than
Brutus.

After some time, Brutus finally reveals that his
wife, Portia, has committed suicide.  (She is also Cassius's sister, making Brutus and
Cassius brothers-in-law.)  Obviously, the news of his wife's death has upset Brutus,
making him more likely to argue.  Further, the stress of the situation Brutus and
Cassius are in weighs heavily on each man's mind.  Ultimately, the two reconcile and vow
to continue their fight for Rome.

(It is also important to
note that Brutus and Cassius disagree with regard to battle strategy at the end of 4.3. 
Brutus feels the best strategy would be to meet the enemy at Philippi, whereas Cassius
thinks it would be better to allow the armies to rest and wait for the enemies to come
to them.  Ultimately, though Cassius agrees to yield to Brutus, audiences learn, once
again, that Cassius's wisdom and ideas would have worked out
better.)