Sure!
The FI, to my ear, sounds more analog, whereas the C4 sounds more digital. The FI seems to have a certain fatness to it that I love. I realize they are both digital, and may even be able to sound similar or exact with tweaking, but out of the box, I'd say the FI has a different feel to the tone that I dig.
Strengths of the C4- loads of flexibility, can store up to 128 presets internally, can enter the PC editor with a simple USB cable, small footprint, can easily sample community patches before downloading, can easily be a synth or envelope filter, tracking is really good, can take the place of several other pedals with some patience.
Weaknesses- needs additional equipment in order to access all 128 presets. Unable to accept PC messages at the moment (CC only), somewhat difficult to fully replicate sounds from the Manta, despite being billed as a much better version of that pedal (which is why I still have both on my board), steep learning curve.
Strengths of FI- big, fat tones, 99 presets available from the get go, many being usable immediately, somewhat simple to modify existing patches to get what you're looking for, rather than starting from scratch, no extra equipment needed to access the presets.
Weaknesses- large footprint (unless you rehouse, like I and many others have), accessing the PC editor requires a MIDI interface, which requires disconnecting from a MIDI controller while editing, making it a bit more of a tedious process, presets are available, but scrolling through, from the pedal, can take some time, sampling a user patch isn't difficult, but a couple extra steps comparatively speaking, editor is, to me, a bit more daunting, and may require more learning in order to maximize it's full potential, steep learning curve.
I'm sure most can get by with one or the other. I find they both have their place on my board, at least for now. I don't know if one is better than the other. They're two sides to the same coin. Both require time and patience to get the most out of it, and spending time learning about synthesis in general is a good idea. That may turn some people off, or make them think either pedal is broken or not worth the hype. I've barely scratched the surface of either, but still find both to be integral on my board. YMMV.