Yes, you can upgrade from Lr 4 to Lr 5,first you need check the system requirement for Lr 5. Refer: System requirements for Adobe Lightroom for Mac and Windows OS
Once everything is fine, you can go head and purchase the Lr 5, however Lr 6 is latest version so we suggest you to go for latest version.
Products Link: Products
Download Link: Download Photoshop Lightroom
Hope it helps,