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.
Note that all of these 3 projects have the same primary author. But the latter 2 were created to correct issues in the former. There is no reason to continue using log4j (no longer under active development) on any new projects.
Also sl4j is a logging abstraction layer it is not a logging framework, it is used together with a logging framework (like logback or log4j).