What are the four subplots in "The Merchant of Venice"?

If you consider that the main plot of The Merchant of Venice is the contract between Antonio and Shylock and its repercussions, then other story threads can be considered subplots, such as:
•Bassanio's voyage to find and marry Portia, which to modern eyes seems a lot like gold digging.
•Jessica's relationship and escape from her father, Shylock, to elope with Lorenzo. This, again to modern eyes, is perhaps the most poignant part of the story. She will have to renounce her religion to marry Lorenzo, and her departure causes Shylock anguish (and not just about the missing ring).
•Portia and Nerissa's journey, disguised as men, to Venice to pursue Bassanio and Gratiano, ending in the trial.
•The comic interludes with Shylock's servant Launcelot Gobbo, including the scene with his father, Old Gobbo. To Shakespeare, however, this would have been less of a "subplot" in the way we mean it than a way to incorporate slapstick and physical humor for the clowns in his company, which the audience expected to see in every play.

In no particular order, here is a list of five subplots that exist within Shakespeare's The Merchant of Venice.
There is a subplot involving a casket and Portia's suitors.  Portia's father is not going to allow any man to marry Portia unless that man can correctly choose the casket that has Portia's portrait.  All potential suitors have failed this test.  All except for Bassanio.  
There is a subplot involving the contract that exists between Antonio and Shylock.  Antonio's money is currently tied up, so he needs to borrow some.  Antonio needs a moneylender, and his moneylender winds up being Shylock.  There is definitely a lot of antagonism between these two characters, but Shylock does agree to loan Antonio money interest free; however, Shylock demands a pound of flesh if Antonio can't repay the loan.  
A part of the previous plot is the entire court case.  I'm not sure if you want to call it a separate plot or not, but it does involve Portia dressing up like a man in order to be Antonio's defense attorney.  
There is a subplot that involves a ring, Portia, Nerissa, Bassanio, and Gratiano.  
A final subplot is the love plot that is about the multiple relationships and marriages going on among Bassanio, Portia, Nerissa, Gratiano, Lorenzo, and Jessica.