I've just been promoted from platinum league to diamond and master pretty fast (in 4 vs 4). This was totally amazing for me but I'm afraid I will be demoted again to a lower league as fast as I came higher.
If I don't play any 4 vs 4 anymore, will they demote me to a lower league automatically or do they only do so, if I play?
For Master, Diamond, Platinum, Gold, Silver, and Bronze leagues, you will not be demoted for inactivity. (Inactive players are demoted out of Grandmaster league.)
However, your league membership only lasts for a season. And, if you are inactive for a full season, then play in a later season, you are considered a "new player" for league purposes.
Edit: If you skip an entire season, your MMR will be reset.
MMR persists from one season to the next for each game type as long as a Seasonal Placement Match was played for that season. If a Seasonal Placement Match was not played last season, then MMR and uncertainty are both reset to their default values and the system effectively "forgets" about that player.
http://wiki.teamliquid.net/starcraft2/Battle.net_Leagues#Hidden_.22Matchmaking_Rating.22_.28MMR.29
I would add one more point: if you don't play for more then couple of seasons your skills degenerate severely so you anyway downgrade. Restore playing, push hard, and you can climb back. But it is really good you will be downgraded after inactivity as you will get some fun restoring your skills and competing with lower player guys.
