In Scene 4, Portia appears to know that Brutus is involved in a plot to kill Caesar, although the play does not include a scene in which Brutus gives her this information.
"Julius Caesar" by William Shakespeare
I do see it as a weakness because it's unclear is scene IV if Portia is worried because her husband is putting himself in a dangerous situation or if she's worried because her husband is about to kill a leader she loves. Remember Caesar treated Brutus like a son, so I'm sure Portia and Caesar were close too. I would like to see a scene added that explained how Portia feels about the conspiracy. That would also give more meaning to her suicide in Act IV and not make it so sudden and surprising.
I would write a scene in which Portia begs Brutus to not be a part of the conspiracy. Portia could insist that Brutus have no part. She could threaten to kill herself and try and stop him from assassinating Caesar.
Portia could convince Brutus that he should not allow men such as Cassius to persuade him. She could point out Cassius' lean, hungry look. She could convince Brutus that Cassius himself is power hungry.
If this doesn't convince Brutus, she could threaten to leave Brutus. She could begin packing her bags directly in front of Brutus. Perhaps his love for Portia could change his mind.
Obviously, Brutus loves Portia and that is exactly why he conceals the conspiracy from her. He is concerned that she could convince him to not be a part of the conspiracy.