A recent change in vB licensing arrangements and a good number of forums remaining on 3 series might mean that 3 series could remain supported for some time. I certainly hope so. I'm not moving useful forums to 4 series having tested it and kept an eye on it, but I'm not moving them elsewhere either as all my URLs will change.
Others must feel exactly the same as a mess up in a 3 series upgrades got a mention on the WebmasterWorld homepage. For the record it revealed database access credentials rather than passwords to the admin account. Leaves you open to more damage, but sensible server management (ie not allowing database access from remote machines) could have reduced risk. Not upgrading for a few days after an update is the best security measure - there are always patches.