Blocking row 3+4 is MANDATORY below a certain amount of passengers.
The blocking of rows 3-8 and the last 4 is an OPTION to slightly shift the balance limit (usually depending on the amount of luggage / cargo in the holds).
So if the loadsheet shows a nicely balanced aircraft with just a few passengers it is always possible to block just rows 3+4.
There has actually been a change in procedures a few months ago - now it is always possible to sit in the first two rows as it often led to passenger complains when it was not possible to sit there.