OPx-4 is an FM synthesizer from AIR Music Technology, part of the MPC Instrument Collection alongside Fabric, Studio Strings, Stage EP, Stage Piano, and Organ. It is built on the operator architecture FM synthesis uses, where a carrier's waveform is modulated by one or more operators, and its name encodes the arrangement: an operator matrix where four operators can be routed in multiple configurations. That structure is what produces FM's signature timbres — electric pianos, bells, marimbas, glassy pads, and punchy digital basses — as well as the metallic and inharmonic content FM is valued for in sound design.

Its interface orientation is toward clarity: a visual operator matrix with draggable routing, so the algorithm is seen rather than selected from a list, plus individual operator controls for frequency, level, and envelope. Instruments in this class generally add multiple filter types, LFOs and modulation envelopes, an arpeggiator, and effects including chorus, delay, and reverb, along with preset browsing by category. It suits synthwave, pop, electronic, hip hop, and cinematic production, and pairs naturally with Stage EP for electric piano work. Requirements are Windows 10 and 11 or macOS 13 to 15 (Intel and Apple Silicon), in VST 2, VST3, AU, and AAX formats, with full support for MPC standalone and desktop systems, and registration with the manufacturer plus an internet connection to authorise. Confirm the operator count, preset library, and effects list on the developer's product page.

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