In Act 4, Scene 2, Messala already knows that Portia is dead, but at this point he does not know if Brutus has already heard about his wife's death. He is certainly not going to tell Brutus of Portia's death if Brutus already knows. For some strange reason Brutus claims that he knows nothing about his wife's suicide, although he has just told Cassius that he knows all about it.
Impatience of my absence,
And grief that young Octavius with Mark Antony
Have made themselves so strong--for with her death
That tidings came. With this, she fell distraught,
And, her attendants absent, swallowed fire.
It would seem that Brutus wishes to impress Messala and Titinius with the strength of his philosophy. When Messala tells him:
Then like a Roman bear the truth I tell;
For certain she is dead, and by strange manner.
Why, farewell, Portia. We must die, Messala.
With meditating that she must die once,
I have the patience to endure it now.
And Messala says with admiration:
Even so great men great losses should endure.
That reaction seems to be exactly what Brutus wants and expects. He knows that Messala and Titinius will both spread the word that Brutus, the great philosopher and great leader, accepted Portia's death with great courage and fortitude. Brutus has many good qualities, but Shakespeare shows that he is something of an egotist, not unlike Julius Caesar himself. Perhaps all great men are also great actors. Brutus's stoicism may be ninety percent sincere and ten percent fake.