I've been playing Paradox games since the original Europa Universalis, and yes, they do continually update their games, even some of the older ones. They've actually been criticized extensively for this ("Why can't they finish a game
before they release it!") by people who don't understand that the changes are often driven by player requests - as people play the game, they ask for certain features to be added or tweaked to improve playability. The players love it.
Haven't played EU4 yet - still trying to play a game of CK2 to completion.