It is my understanding that when you put the EQ6 on a computer it is counted as one install. if the computer crashes and you don't change anything out like hard drive, etc, only fix it. You can re-install on the same computer and it doesn't use an install. You can check with EQ about this. Also, if you do use all 4 of your installs, you can pay $40 and get additional install.
If you are buying a used software package, ask for the #, call EQ and they will tell you how many installs are left.