The minimum requirement for CDH-4 and CDH-4 is 4GB RAM and CDH-5 requires 8GB of RAM. To run CDH-3 or CDH-4 you should have 8GB of RAM in your system.
If you have 4GB RAM system, you can either upgrade your system to additional 4GB RAM or you can install Apache Hadoop in Ubuntu server which only requires 1-2 GB of RAM.
Hope this helps.
My memory is onboard so it would be kind of expensive to upgrade my RAM. Is there a guide I can follow to install Apache Hadoop in Ubuntu server?
You can install the apache hadoop in ubuntu server using following document: