What fears does Juliet reveal in her soliloquy (speech) in Act IV Scene iii?
I am assuming you mean Act IV scene 3 as you didn't state the Act in your question. I have edited your question for you. This soliloquy of Juliet comes as she is just about to take the potion that Friar Lawrence has given her and supposedly fall asleep to be revived by her beloved Romeo and successfully trick everyone into believing that she is dead and therefore escape marriage with Paris.
In this speech we cannot help but admire Juliet and her strength of character. She comes up with a number of reasons why she should not take the potion, suspecting Friar Lawrence of giving her poison to avoid blame for marrying Romeo and Juliet or if the potion doesn't work well and she wakes up early in the tomb before Romeo gets there and is driven mad by fear.
However, drinking the potion represents Juliet taking her life into her own hands. She is fully aware of the dangers, yet she chooses to drink the potion anyway, attempting to beat the forces that are so implacably opposed against her union with Romeo. Note here that drinking the potion is an obvious foreshadowing of Romeo drinking the poison.
where is juliet when she takes the portion
She fears that the potion wont work and she will have to marry paris, that the friar poisoned the sleeping potion so that he wouldnt be looked down upon for marrying her to Romeo, that she wakes up before Romeo gets to her, that she suffocates in the vault; she also fears having to see her dead ancestors, including Tybalt and his bloody corpse, and their ghosts, and she fears of having to smell and hear death/ghosts, and that she will go crazy from the memory of waking up in a tomb.