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In the fourth movement Berlioz' use of orchestration is the dominant force in the expression of the programme. For example: the striking effect of the timpani at the very beginning of the movement was due to the sheer number of instruments (five in all) and the number of players (three). This was quite unprecedented in orchestral music of the day. However Berlioz' highly detailed performance directions gave such specific instruction that his programmatic intentions could not be ignored. Berlioz goes as far as to specify the drumsticks to be used: "Baguettes d'éponge". The use of the rumbling timpani, low end military brass instruments and the muted sponge-headed drumsticks in the opening highlight "the procession advance[ing] to the sounds of a march" as the distance of the music is effectively captured through the staggered entry of the less easily heard instruments later in the score. However the instrument that primarily captures the military-band atmosphere is the new valved cornets (cornetta a pistoni). Berlioz' interesting addition of cornets perhaps highlights the "brilliant" orchestration he refers to in the programme. Berlioz cleverly emphasises the familiar marching band with the later addition of the bass and snare drums. The audience association of the mass drummers and marching band also connects with the programmatic execution: "the fatal blow". .
Berlioz also presents the "dark and savage" nature of the march to the scaffold in the music of the fourth movement. The pizzicato of the low divisi double basses (very innovative for its time) and the cellos reinforce the sound quality of the reverberating drums adding to the atmosphere of dark melancholy that builds to the climactic point of the movement. Prefacing "the fatal blow" Berlioz reintroduces the Idée Fixe. The solo clarinet's statement of the first phrase of the idée fixe, the artist's "final recollection of love", stands out explicitly from the relatively dense orchestration seen beforehand.