This species is also called "ligature," or "syncopation." Two half-notes are set against each whole-note, but the half-note on every downbeat is tied to the half-note that came before it. So, to the ear, we've got whole notes against whole notes, with the counterpoint offset by half a bar.
I've only done the Dorian exercises, so far. My impression is that the rules governing 4th species are much more confining that the previous species. Several times I found myself in a position where I had only one option in three or four consecutive bars. The impression of confinement is supported by the fact that my solutions are identical to Josephus's for big stretches.
I think 4th Species will sound wonderful when it's put together with a third voice, but with only two voices, it's a little bit dull: