I own both a B1 Four and a B1Xon, and personally I prefer the sound of the B1Xon.
Also much more flexible in terms of tweaking the effects, and unlike the B1X Four the expression pedal on the B1Xon can be set to control any 1 parameter of any effect in each single patch, at whatever direction and range you desire.
It's beyond me why they made the newer version less flexible, but I suppose most people might prefer simplicity over flexibility.
With the B1X Four you are limited to 4 parameters for by far most effects, on the B1on and B1Xon you have access to 6 or more tweakable parameters for most effects, and rather than being able to assign the expression pedal to control any parameter within each single patch, as you are able to with the B1Xon, with the B1X Four you are limited to about a handful of fixed expression pedal specific effects.
That said I wouldn't use any of these units for pitch, drive or amp/cab modeling, which in my opinion sucks big time (especially the pitch/octave effects are truly horrible, but I suppose these effects can still be used for special deliberately bad/digital special sounds/effects), but other than that the rest of the effects comes in the quality category from decent to genuinely excellent (with the right amount of tweaking), most are actually pretty good, and as said, some are genuinely great.
Generally the EQ effects are as good as any, most of the delay and reverb effects are great, as are the chorus effects, vibrato and tremolo models are a bit basic, but other than that actually pretty great, and there are a few great compressor models in these units too.
finally in favor of the B1on/B1Xon, firmware hacks exists, which means you are able to load any digital Zoom effect model from the same generation into the pedal (that is all the effects from the G1on/G1Xon, MS-50G, MS-60B and MS-70CDR) into the pedal, which is not, yes at least, possible with the Four series pedals.