The simplest answer is this: Predict that the value of the 4th variable is "foo" . Put "foo" in that variable in the training set, and the machine will correctly predict that all the 4th variables will be "foo". Problem solved.
If that solution will not satisfy the person that gave you the assignment, then we need to actually understand what the data is, and what the expectations are.
You have given us very little to answer your question with. Here's the top five questions that came into my head...
(1) What ARE the values?
(2) Do they have a known or expected relationship?
(3) Are they categorical or numerical, continuous or discrete?
(4) How many examples of each do you have?
(5) Who asked you to do this?
(6) Have they told you how to predict the value?
(7) How will they know if the value you predict is right?
Okay, I got up to seven questions before I realized how vague the assignment was and added the intro paragraph based on the critical importance of the last three questions.
I can now confidently say the value of variable 4 is definitely "foo"... Or possibly "bar".
