This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I don't think there is a specific answer to this. You should have solid knowledge of the technologies you've worked with. If you worked on a web app, I think this means knowing servlets and JSPs well. (even if you worked with a framework.)
You should have the ability to code algorithms and test them. You should be able to communicate. etc
You might look at job listings to see what employers in your area want from someone with 4 years experience.