The ARP Quartet is identical to the Italian made Siel Orchestra. In fact, it is the Orchestra! ARP bought, relabeled and sold it as the Quartet without changing it much. It's a 4-part orchestral synthesizer with 4 sections: Brass, Strings, Organ and Piano. These 4 tone color families can be played individually or two can be simultaneously played. They are also individually tweakable. Selected presets will light up LEDs to show you which sliders and buttons can be used to modify the current section. The synth itself is very limited however, and there's no external control, no pitch/mod wheel and few edit parameters. But it sounds very good and it is, after all, a classic analog synth.
The Brass section creates Trumpet and Trombone sounds with independent attack and sustain controls. The String section creates Cello and Violin sounds with independent attack and sustain controls. The Organ section is 2-voice and has a Celeste sound available as well. The Piano section offers two types, traditional or Honky Tonk. The Quartet has been used by 808 State and Massive Attack for some those great string sounds.
Demos & Media
Images from Perfect Circuit Audio.