all 3 are actually rather different
q tron is a envelop filter, in the vein of 'auto wah' type, where your playing dynamics will control the "wooooooaaaap" depth/strength. Think of geetar playing in a porno soundtrack
zvex seeker is a step sequencer filter where the guitar signal is as if being "chop up" as the signal goes thru the 4,6 or 8 filter in series. Think of frank zappa, ship ahoy(though thats kinda different pedal, a random sample and hold effect)
fm4 contained the above and much much more, but its a modeller. Theres different in the quality of the sounds you will get. This, is good, if you want more of out there sounds with some of the setting within, totally having bad tracking, but having its own unique id)