A curious passage occurs twice in the Missa Solemnis, during the Benedictus. Beethoven asserts the submediant (vi) strongly, with dotted rhythms, brass and timpani. The viiø/V is then asserted in the same manner.
Normally, this formula would proceed to the appearance in the bass of the fifth scale-degree, whether it’s accompanied by the dominant or the tonic six-four (which is, in reality, also the dominant, but retarded by two suspensions).
In the Beethoven, however, the viiø/V gives way to the subdominant – the IV! This is exceptional because the traditional formula is not an arbitrary construct. When we hear, in the bass, the sixth scale-degree followed by the raised fourth scale-degree, we expect the fifth scale-degree. The former two scale-degrees serve as a double neighbor-note configuration to the latter; they “close in on” the dominant. Especially since the fourth degree is chromatically altered to achieve an upward-tending inflection.
But instead of the dominant, or the tonic six-four, Beethoven gives us the subdominant. The raised fourth descends to the diatonic fourth.
I’m still trying to decide if this really works or not. I can’t see any longer-range justifications for this peculiar harmonic turn. I’ll let you know if I figure it out.