I haven't done one yet, but plan on building a 4 piece 10 wt as well for a project this winter. The extra work versus a 2 piece that I can see is:
2 extra ferrules to wrap
2 extra sections to spline
Maybe some time spent playing with the alignment of the sections when checking the spline of the assembled rod.
Possibly having to play around with the guide placement vs ferrule location.
Finishing 2 extra sections. I only do a section at a time to prevent gluing them together
My guess is maybe an hour or two difference in the time required to build the rod.