Just ordered a new X4 and I am wondering if it is possible to run the line out of my FP33 into the X4 to gain two more mic inputs. If this works, am I better off doing this than splitting channels on the X4 when I need more inputs?

I think it might be easier when you need to combine the mixers to have the 33 as the 'master'. From what I remember, the X4 doesn't have any kind of aux in. This page (http://www.sounddevices.com/notes/mixers/mixpre/interfacing-the-mixpre-or-302-with-a-shure-fp33/) documents connecting a mixpre and a 33, which might give some idea how best to connect the wendt.

Kevin,
Yes you can use the FP 33 to do a submix to the Wendt X4. . Of course if you had gotten a Wendt X5 you could link it to the Wendt X3 like you can link the FP-33 to another FP 33 to get six inputs and 4 xlr outputs.