Each upgrade of Lr is a stand-alone version, i.e. it is not a "patch" for the older version.
Buying or upgrading differ only in the license and in the price. In each case you receive / download a full version but if you have bought a prior version of Lr, you qualify for the upgrade price.
When you the install Lr 4.3 it will search on your computer for a prior version of Lr. Be prepared to be asked to enter the serial # of your Lr 1.
So in your case download Lr 4.3 from Adobe's website as "upgrade" from here (if you reside in the US): http://www.adobe.com/products/photoshop-lightroom/buying-guide-upgrade s.html