Hmm. I am not impressed with the two C4 samples. In my opinion Quatschmacher has better patches of bass synth.
Which exact two samples are we talking about?
Side note: I have listened to two complete presentations of the factory default sounds in the FI; the one at Panda' home page, and one on YT, where a man played licks through them all. In addition to that, I also checked out many other FI demos.
For the C4, I think I have seen the major demos on YT, where also various patches from the user community were demoed.
The impression I (still) have, is that the C4 sounds in general have more Attack/"slam", and are a little "dirtier". The unfiltered square waves provided by the very helpful @Quatschmacher display the higher amount of harmonics in the C4.