The difference between Portia’s behavior in Scene 1 and Scene 4 of Act 2 of Julius Caesar can be explained by a few lines spoken by Brutus towards the end of Scene 1 of that act.
O ye gods,
Render me worthy of this noble wife!
Hark, hark, one knocks. Portia, go in awhile,
And by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,
All the character of my sad brows.
Leave me with haste.
Shakespeare does not show a meeting in which Brutus reveals all his secrets to Portia. The playwright felt this would be redundant because the audience already knows everything Brutus could tell her. In Scene 4, however, Portia obviously has been fully informed about all that her husband is thinking and planning. Now she has become, in effect, a co-conspirator. She was worried and distressed in Scene 1 while she was still innocent and ignorant. In Scene 4 she is even more worried and distressed. She is worried about her husband but also worried about herself. If his plot against Caesar should fail, then Caesar could be expected to enact reprisals against everyone connected with it, including their families. Brutus had been trying to keep her in ignorance for her own protection. If the plot failed, it might be to her advantage not to have known anything about it. He also wanted to spare her the anxiety he was experiencing himself.
It seems like a minor miracle that Caesar hasn’t been warned that there is a very serious and well organized plot against his life. His own wife Calpurnia sensed intuitively that men were planning to kill him and begged him not to leave home that day. There are about thirty conspirators. One of them could be a spy. Or one of them could reveal all their secret plans to an outsider, just as Brutus has revealed them to his wife. Benjamin Franklin once wrote:
Three can keep a secret if two of them are dead.
Portia is afraid something will go wrong. She voices her fear at the end of Scene 4.
I must go in. Ay me! How weak a thing
The heart of woman is! O Brutus,
The heavens speed thee in thine enterprise!
Sure the boy heard me. (To Lucius) Brutus hath a suit
That Caesar will not grant. (Aside) I grow faint!
Run, Lucius, and commend me to my lord.
Say I am merry. Come to me again.
And bring me word what he doth say to thee.
Scene 4 is intended to heighten the dramatic tension. Portia's fears are communicated to the audience. They are not sure the plot will be successful. They are back in the past before the assassination has occurred. Then Act 3 begins, and the audience assumes they are about to witness a reenactment of a famous event in history, the assassination of Julius Caesar. The main interest of this play is that it makes the audience feel like time travelers. They are back in the past reliving these historic events.