In fourth-species counterpoint, the counterpoint line and cantus firmus both move once per bar, but they are rhythmically offset from each other by a half note. (Think syncopation on the bar level.) The counterpoint line will be notated in half notes, with each weak-beat half note tied across the bar line to the following strong beat. This arrangement means that in pure fourth-species counterpoint, the two lines always move in oblique motion. It also introduces a new kind of dissonance: the suspension.
The suspension is an accented dissonance, meaning it always occurs on strong beats. Like the passing tone and neighbor tone dissonances, it is always preceded and followed by harmonic consonances.
A suspension figure has three parts:
Use dissonant suspensions as much as possible in fourth species. Not only are they the characteristic sound of fourth species, but they sound nice, and proper use of them in fourth species will prepare you for the use of both suspensions and dissonant chord tones in later composition and arranging work.
Suspensions are categorized according to the intervals of the suspension and resolution tones above/below the cantus firmus. A 7–6 suspension, for example, includes a strong-beat suspension that forms a seventh with the cantus, which resolves down by step to a weak-beat tone that forms a sixth with the cantus.
Possible dissonant suspensions above the cantus firmus are 7–6, 4–3, and 9–8. (These are the only options that start on a dissonance and resolve to an allowable consonance.) Possible dissonant suspensions below the cantus firmus are 2–3, 5–6, and 4–5. (7–8 is theoretically possible, but it sounds bad. Don't use it.)
Treat suspensions in fourth species the same way you would treat their intervals of resolution in first species. In other words, do not use two 9–8 or 4–5 suspensions in a row (since you cannot use two octaves or two fifths in a row in first species). Use 7–6 and 4–3 (above) or 2–3 and 5–6 (below) liberally, but no more than three times in a row (like thirds and sixths in first species).
Following the same principle, do not use the "consonant suspension" 6–5 twice in a row, since its interval of "resolution" is a fifth. In fact, avoid any configuration that would create two fifths or two octaves on consecutive weak beats in fourth species (called "after-beat" fifths or octaves).
Use dissonant suspensions whenever possible. This will create a line consisting mostly of downward, stepwise motion. That is fine. It will also make it hard to direct motion towards a climax. That is also fine. Do not worry about the shape of the line if it is singable and the suspensions are properly prepared and resolved.
If a dissonant suspension is not possible, try to use a tie from weak beat to strong beat. This can be a "consonant suspension," or you can leap up from downbeat consonance to weak-beat consonance. At least one or two upward leaps will be necessary to counteract the downward resolutions in order to keep the line in a singable range.
If neither a dissonant suspension or consonant tied figure is possible, it is permissible to break species (see video demo below). When you break species, follow the principles of second-species counterpoint and resume fourth-species ties as soon as possible. Try not to break species more than once per exercise, and for just a bar or two.
Begin a fourth-species counterpoint above the cantus firmus with do or sol. Begin a second-species counterpoint below the cantus firmus with do. Unisons are permitted for the first and last dyads of the exercise.
Always begin with a half rest.
There is only one option for ending fourth species.
The cantus firmus must end with re–do. Do not use a cantus that ends with ti–do.
The counterpoint will end with a dissonant suspension. The penultimate bar will contain do–ti, and the final bar will contain a whole note do. The do–ti will form a 7–6 suspension above the re in the cantus, or a 2–3 suspension below the re in the cantus. As a dissonant suspension, that do will always be tied over from the previous bar.
In the following videos, I illustrate the process of composing a fourth-species counterpoint above and below a cantus firmus. This video provides new information about the compositional process, as well as concrete examples of the above rules and principles.
Composing a fourth-species counterpoint above a cantus firmus from Kris Shaffer on Vimeo.
Composing a fourth-species counterpoint below the cantus firmus from Kris Shaffer on Vimeo.