Adobe has different upgrade policies in place for different products. Can be mighty confusing to the uninitiated.
Creative Suites have the most draconian upgrade policy - one version back only: you cannot upgrade to CS6 unless you have CS5.
Lightroom is not included in any Creative Suite - but it is part of the Creative Cloud.
Fortunately, if Lightroom is the only software you need, then you can upgrade to Lightroom 4 from any previous version of Lightroom.
http://www.adobe.com/products/photoshop-lightroom/buying-guide-upgrade s.html