Adding Additional Subscription (over 3)

I presently have NIS installed on 3 machines....one of which is rarely, if ever, used.  I recently bought a new machine I'd like to install NIS on but I'm having trouble figuring out which of my 3 machines is the one I'd like to delete NIS from.  Accordingly, if I leave NIS on my three existing machines and if I buy a key for the 4th machine.....will I pay the $39.xx for the one additional subscription and will this additional subscription run on its own 3 year time table?  I hope this make sense.  I appreciate your help and expertise.