Hello Akshay
Its good that you have some prior knowledge of java.
Hadoop is a framework which is based on java programming. It is intended to level up from single server to thousands of machines each offering local computation and storage. It supports the huge data set in a distributed computing environment. Hadoop is written in Java, so there is a need of Java installed on the machine and version should be 1.6 or later.
As you said that you have 4GB RAM and whatever the configuration you have that's enough to work on Hadoop. Hadoop is supported by Linux platform and its flavors. So you have to install a Linux OS for setting up Hadoop environment. If you have another operating system than Linux then in this case you can install virtual machine and have Linux inside the virtual machine
To learn Hadoop basic commands knowledge of UNIX, SQL scripting. Prior knowledge of Apache Hadoop is not required. Hadoop is a booming and on demand technology in the industries. Top Big Data Analytics companies like Google, Yahoo, Apple, eBay, Facebook and many others are hiring skilled professionals capable of handling Big Data. So its a good opportunity for you give perk in your career to learn hadoop from basics to advance level.
To learn
Big Data Hadoop and its ecosystem and work on real-time applications of big data, a professional training from Industry Experts is considered a must-have.