I use M 4's as heights over & to the sides of Magnepan 3.6 planars. They work very well.
Your choice is obviously directional vs diffused. This is only my opinion but even though I have dipolar Maggies, similar in nature to M-L e-stats, I don't like diffuse surround sound. I do want directionality.
You may feel differently
Again, only my personal opinion - dispersed diffused sound will work better as rear surrounds rather than front heights. My reasoning is this - if you have good L/R fronts that image very well for music as well as movies, using bipolar/dipolar heights could degrade the precise imaging in the front soundstage, making it more diffuse. Usually it's surround speakers than get the bipolar/dipolar treatment. but if you are already using a bipolar/dipolar speaker for fronts, then it might not be as much of an issue. But even so, my inclination is still to use monopoles as heights rather than diffuse & make less precise the front soundstage.
There are some speaker companies who disagree, namely Atlantic Technology, who makes a 'z" speaker that is bipolar design, like the FX. I looked at that approach and rejected it as it goes against the philosophy of a defined, more focused front soundstage.
If I did want a diffused surround field, I'd use FX's as rears. But my choice would still be to use the Motion 4's or even Motion 15's as front heights. For me, the 4's physically fit my situation better than 15's because of the height of the Magnepans.
IMO, the Motion 4 can be an excellent height speaker, positioned up high near the ceiling & rotated so the narrow part is at the bottom so that it aims downward. You may have to use an Omnimount kind of ceiling mount rather than the built-in wall attachment but that depends on whether you want to mount up high on the walls or physically on the ceiling away from the walls.
Some Best Buy-Magnolias or other M-L dealers may have FX's wired up for listening. That would be your best option...compare them for yourself. I think our local BB-Mag had them hooked up at one time but not the last time I was there.