From my understanding, the C1 is discontinued and is replaced by the C1+.
I would let Emlid team comment on if navio+ is pin compatible with the C1+ (exception of pin #38 and #40 it looks ok).
(I don’t know if navio+ uses those 2 pins).
I don’t think the XU4 is pin compatible, at least, it does not look like it.
But, it does not stop you to use it as an additional computer on board for sure (just not a NAVIO+ cpu board).
“The 40-pin header pin map is compatible with the ODROID-C1 and Raspberry Pi/Pi 2.
We are working on the WiringPi compatible libraries to simplify development.”