We consider two different lengths here, the simple dimeter and the long tetrameter catalectic.
This means that half of one verse foot goes unfilled with text, that is, is catalectic ([Phi]--).
The anapestic tetrameter catalectic is the most common of the anapestic meters.
As we have said, it is hard to know exactly which metrical position is the catalectic one, so we follow traditional analyses and assume it is the last, since catalexis (like extrametricality) seems to target final constituents rather than initial ones, at least in phonology (Kiparsky 1991).
For one thing, the clearest case of catalexis in Greek stichic meter, iambic tetrameter catalectic, has initial catalexis, not final.
We simply don't know at this point whether the catalectic position is line-initial or line-final.
In the cases at hand we need to define what it is to be anapestic dimeter, anapestic dimeter catalectic, and anapestic tetrameter catalectic.
Moving on to the catalectic version of anapestic dimeter that we find at the end of most dimeter systems, we note that it has one less filled metrical position than we expect a dimeter to have.
We capture this formally by noting that a catalectic meter intentionally violates FILL.
The formalism is to be read `a line is catalectic (C) if it violates the constraint FILL'.
Anapestic tetrameter catalectic is still rhythmically unmarked, but it now has two peculiarities in terms of length: it is twice as long as we would expect it to be (if it were a dimeter) and it has one less metrical position than we'd expect it to have (binary meters always have an even number of metrical positions).