The Line 6 FM4 Filter Modeler is a digital stompbox pedal that provides sixteen distinct models of vintage and monophonic synth effects, dynamic filters, and auto-wahs1. It was part of the same Line 6 stompbox family as the DL-4, MM-4 and DM-4, which MusicRadar's Trevor Curwen described as pedals that "put a range of delay, modulation, distortion and filter effects underfoot"2. Line 6 lists the FM4 among its legacy products and states that "FM4 is no longer made"3.
What the FM4 does
The FM4 belongs to the filter pedal family: effects that reshape the tone by sweeping the frequency content of the signal, from auto-wah and envelope-driven filters to synth-like voicings. Line 6's sixteen models cover three broad families of effect1:
- vintage and monophonic synth effects
- dynamic filters
- auto-wahs
Delicious Audio's Ezra Tenenbaum described the approach this way: "The FM4 re-imagines a number trademark filtering effects from companies such as Moog, Korg and Oberheim"4. Among the preset modes he singled out were "the arpeggiated 'throbber' to the stabbing 'attack synth'"4.
Controls and presets
The layout features six parameter knobs that adapt their functions depending on the selected filter model, plus an expression pedal jack for real-time parameter morphing1.
| Control | What it does |
|---|---|
| Six parameter knobs | Functions adapt depending on the selected filter model1 |
| Mix knob (far right) | Fixed function, unlike the other dials4 |
| Expression pedal jack | Real-time parameter morphing while you play1 |
| Four footswitches | Select the four user-programmable preset channels14 |
Tenenbaum noted that "save for the mix knob on the far right, the three other dials control variables that sometimes change dramatically, depending on the preset mode"4. The expression jack means an expression pedal can sweep a model's parameters in real time while you play1.
Power and connections
The FM4 runs on 9V DC, center negative, and draws 250mA1. Power can be supplied in two ways1:
- four C-cell batteries
- an external power adapter
At 250mA1, the FM4 needs a power supply that can deliver that current; it can also run on four C-cell batteries1.
The pedal is equipped with stereo inputs and outputs1 and uses true bypass switching1.
What reviewers said
Reviewing the pedal for Delicious Audio in 2010, Tenenbaum wrote that "no pedal in its price range ($250) matches the FM-4's smorgasbord of quirky sounds"4. His verdict on where it sits in the Line 6 range: "While the FM4 Filter Modeler is not as user-friendly or universally appealing as the DL4, it follows in the same creative footsteps"4.
Videos
Line 6
Reverb
gearwire
Questions
Does the Line 6 FM4 have true bypass?
Yes. The FM4 uses true bypass switching, so when the effect is off, the pedal's circuitry is removed from the signal path entirely. That matters on a pedalboard because filters and modelers can colour the tone even when switched off, and true bypass keeps the bypassed signal untouched.
Can the Line 6 FM4 run on batteries?
Yes. The FM4 can be powered by four C-cell batteries or by an external power adapter. Note that the pedal draws 250mA at 9V DC, center negative, so a power supply needs to supply that current, and a daisy chain shared with other pedals may not be enough.
Can I use an expression pedal with the FM4?
Yes. The FM4 has an expression pedal jack that allows real-time parameter morphing, meaning you can sweep a filter model's settings with your foot while playing rather than turning a knob by hand.
Is the Line 6 FM4 discontinued?
Yes. Line 6 lists the FM4 among its legacy products and states plainly that the FM4 is no longer made. It remains documented on Line 6's site, including the FM4 Quick Start Pilot's Handbook.
Is the Line 6 FM4 mono or stereo?
The FM4 is equipped with stereo inputs and outputs, even though it is a single stompbox-sized pedal. That gives it more routing flexibility than a simple mono filter pedal.
How many presets does the FM4 store?
The FM4 includes four user-programmable preset channels, selected with its four footswitches. Each preset can hold a different filter model and its knob settings.
Sources (4)
- FM4 by LINE 6, RockBoard PedalPedia, 2026
- Line 6 M5 Stompbox Modeller review, MusicRadar, 2011-06-21
- Line 6 | Legacy Products, Line 6
- Pedal Review: Line 6 FM4, Delicious Audio, 2010-05-13
