As in Second Species, the half note on the first beat is defined as strong, and the second half note is weak. The rhythm of Fourth Species is note-against-note, offset or delayed by one beat. The weak beat in the counterpoint is always tied across the bar line and may become a Suspension when the other voice moves on the strong beat. If it is a consonance when the other voice moves, it is referred to as a Syncope rather than a suspension. Fux referred to tied notes as “Ligatures.”
Fourth Species consists of counterpoint with Suspensions and Syncopations. A suspension is a note that is held across a strong beat, and when the other note in the counterpoint moves it becomes a dissonance. The suspension is prepared (preceded) by a tied common tone and resolves down stepwise to a consonance. The three notes in the counterpoint that create the suspension are the Preparation, the Suspension, and the Resolution (PSR).
The Preparation is a consonance, may occur on either a strong or weak beat, and is equal to the suspension in duration. The Suspension is typically a 4th resolving to a 3rd (4-3), a 7th resolving to a 6th (7-6), and less often a 9th resolving to an octave (9-8). It happens on the strong beat in the upper voice. The primary type of Suspension that occurs in the lower voice is the Bass Suspension, consisting of a 2nd to a 3rd (2-3). It is an inversion of the 7-6 suspension. The Resolution is an imperfect consonance, not a unison, and is always step below the suspension itself. A 4-5 suspension in the lower voice is very rare.
When the same rhythmic formula occurs between the two parts, and the intervals are a 6th to a 5th (6-5) between them, it is referred to as a Syncopation rather than a Suspension. This is because the intervals are both consonances. As in Second Species, it is permissible to leap to and from a consonance. This is often done to reposition the voices for a suspension. The 2-1 and 7-8 sequences of intervals are avoided.
As you write and analyze Fourth Species, you will find that certain combinations of intervals recur. There are a limited number of patterns, depending on the movement of the Cantus Firmus. When writing the CP in the upper voice there are two obvious patterns. If the CF steps upward, an octave in the CP becomes a 7th, which must resolve down to a 6th. If the CF steps down, a 3rd becomes a 4th in the counterpoint, which must resolve down to a third. It is common to create chains of consecutive 7-6 and 4-3 suspensions over stepwise motion in the CF. Ending with a Clausula Vera, as all these examples do, the 7-6 in the penultimate measure resolves smoothly to the octave.
Fifth Species combines all the other species. Generally, common time is used, and no more than two consecutive measures with identical rhythmic patterns are written. The use of parallel perfect intervals is determined by those governing the species used at any given time.
de Lassus: Oculus non vidit (1577)