LTspice IV can be downloaded from the LTC website http://www.linear.com. A direct link to the distributed file is http://ltspice.linear.com/software/LTspiceIV.exe. The file LTspiceIV.exe is a self-extracting gziped file that installs LTspice IV as it extracts.
LTspice IV is updated often. After LTspice IV is initially installed, you can use a built-in update menu command that will bring your installation to the current revision level if you have access to the web. The update process will first download a master index file from Linear's website that has the size and checksum of each file in the distribution. If there is a file missing, of a different size, or a difference between the local checksum and the one from the index file, then that file will be updated automatically. Component databases are merged in the update process so if you've added devices to your installation, those additions won't be lost when you run the automatic update utility.