What is Portia worried about as she questions the Soothsayer?

One wonders where some of the attitudes towards women expressed in the play come from. Not only are there just two female characters in the play, Portia and Calpurnia seem to have accepted their lower social standing as women. In this case, Portia believes her sex has left her with character flaws.

This short scene is intended to show that Brutus kept his word to Portia that he would share all his secrets with her. In Act 2, Scene 1, he says:

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 charactery of my sad brows.
Leave me with haste.

Shakespeare evidently did not want to write a long scene in which Brutus would be telling Portia everything about the assassination plot that the audience already knew. Yet Shakespeare must have felt the need to show that this conversation had occurred. 

Scene 4 also serves to build up more suspense. Portia is afraid that something may go wrong. And her fears are communicated to the audience, who may not have suspected that the conspirators were going to have any trouble carrying out their plan. They see that killing Caesar is not going to be a piece of cake, in spite of the fact that there are so many armed men all pledged to carry out the assassination. Caesar has a lot of friends around him. Artemidorus, as we see in Act 3, Scene 1, tries to warn Caesar that his life is in imminent danger and names names in a letter which Caesar refuses to read immediately as requested.

Portia is like Caesar's wife Calpurnia in having premonitions of grave danger. And, in fact, Portia's premonitions prove to be equally valid, although her husband's problems arise after the assassination plot has succeeded. The disaster Portia intuitively senses comes when Antony makes his funeral speech and stirs up the plebeians to mutiny against the conspirators. So both Calpurnia's and Portia's premonitions of danger prove correct.

*Women are like that they don’t acquire knowledge of people we are for that they are Women just born with a practical fertility of suspicion that makes a crop every so often and usually right they have an affinity for evil for supplying whatever the evil lacks in itself for drawing it about them instinctively as you do bed-clothing in slumber fertilising the mind for it until the evil has served its purpose whether it ever existed or no.
*—William Faulkner, The Sound and the Fury

Portia says, "O Brutus, The heavens speed thee in thine enterprise!" to herself. Then she is afraid Lucius overheard and may have guessed the truth. She is having a dreadful time keeping herself under control and hiding her secret knowledge. She tells Lucius, "Brutus hath a suit That Caesar will not grant" to explain what she meant when she said, "The heavens speed thee in thine enterprise!"

In Act 2, Scene 1, Portia begged her husband to tell her what was troubling him, and he finally promised to share all his secrets. Her behavior in this scene shows that he kept his promise and she knows what Brutus and his co-conspirators are planning to do that morning. She wants Lucius to go to the Capitol to see what happens and to report back to her. But she cannot tell him what she really wants because she cannot reveal her secret knowledge to anyone. When she tells herself, "How hard it is for women to keep counsel" (i.e., to keep a secret), she is expressing Shakespeare's opinion of women, which may or may not be accurate. Obviously Portia cannot go to the Capitol herself because women, especially women of her status, did not do such things. Now that she knows all her husband's secrets, she is concerned about him, but she is also concerned about her own welfare. If the assassination attempt should fail, she will lose her husband, and she could also lose her home and all her possessions. She has inadvertently made herself a co-conspirator, because the assassination has to succeed--or else!

Just as Calpurnia has had premonitions about Caesar's assassination based on dreams influenced by clues picked up by her unconscious, so Portia has had premonitions about Brutus's involvement in a plot against Caesar based on clues she has picked up from her husband's recent strange aloofness and distraction as well as from her observations of Brutus's many recent late visitors. Portia knows something is wrong but does not even understand what it might be. When she tells Lucius to take note of what suitor press on Caesar, she is almost visualizing what is going to happen: the setting at the Capitol will allow enemies to press around Caesar and stab him to death. She is becoming another seer like the Soothsayer.