In Act 2, Scene 4 of Julius Caesar, Portia is worried. Why?

In Act 2, Scene 1, Portia is begging her husband Brutus to tell her what is going on. Brutus is acting extremely strangely, and he is receiving all sorts of mysterious callers at odd hours. He does not want to tell her that he is taking part in a conspiracy to assassinate Julius Caesar because that would make her a co-conspirator. In other words, if the assassination attempt should fail, he would undoubtedly get killed along with all the others, but Portia might be spared and allowed to keep her home and possessions if it were felt she knew nothing about her husband's plans. However, Brutus finally succumbs to her insistence that she has a right to know all his secrets because she is his wife. Towards the end of Act 2, Scene 1, he agrees to tell her everything, but not right then and there because he has visitors to consult with. He tells her:

Portia, go in a while,
And by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,
All the charactery of my sad brows.
Leave me in haste.

We never hear Brutus explain anything to his wife. But in Act 2, Scene 4, it is quite obvious that he has told her everything, just as he promised. Shakespeare did not feel it necessary to show Brutus imparting all this information to his wife because the audience already knows it. There is a big difference in Portia's feelings and behavior now that she had learned the terrible truth. This only adds to all the other tension Shakespeare has been building up as a prelude to the actual assassination. Portia knows that her husband is involved in an extremely dangerous situation which could cost his life, and perhaps even her own. As a woman, she is confined to her home. She can't go to the Capitol to see what is going on. She can only send a messenger--but she can't tell the messenger, Lucius, what she wants him to do. She can't trust Lucius or anybody else. She feels she must not do or say anything that would reveal her guilty knowledge, but she is naturally anxious to know whether the assassination attempt will come off successfully and, if so, what will happen after that. 

She has in fact become a co-conspirator. She wants Julius Caesar to be killed. She may not have wanted that before, but Brutus has brought her into the conspiracy by telling her everything she wanted to know.

There are many men in this play. Shakespeare welcomes an opportunity to show a couple of female characters just for the sake of contrast. One is Caesar's wife Calpurnia, who has a good role as she tries to persuade her husband to stay at home because of her foreboding dreams. The other is Portia, who appears in several scenes and has the leading part in Act 2, Scene 4. Shakespeare undoubtedly had males in his company who specialized in being female impersonators, and he wanted to make some use of them.