No if anything, 4 piece is easier. The shorter sections are easier to handle, and you will not have to align a bunch of guides on a single section of rod.
(If you don't want to read the rest of this post, my whole point is that spine is not as important as some people would lead you to believe and you have choices in how to treat the subject. There is no wrong way to build in respect to spine. Your rod will cast fine.)
Do not get too wrapped up in the whole spine thing. Your blank is likely to be somewhat crooked. Assembling the rod so as to have the natural curve of each section aligned and then wrapping the quides along either the inside or outside of the curve is just as valid of a strategy as building according to spine IMO. You'll have to decide for yourself is you want a wonky rod built according to spine or a straight looking rod. Personally, I base my decision on how crooked the blank is. If it is pretty straight I will go ahead and find the spine. If it is crooked I align the guide along the curve so as to hide the imperfection.
Its a nice touch to be able to say you built a rod according to the blanks spine but the reality is that there is no noticable affect on the finished rod. On the other hand anyone can pick up your rod and see if each section is curving off in every direction.
By the way, I have not heard of any major manufacter that builds according to spine. Not Sage, not Winston, not Scott.
Also, there are no perfectly straight blanks. Don't worry about it unless your blank is really bad.
And to answer the original question, yes finding the spine is harder on a 4 piece but is really only difficult for the two heaviest butt pieces. Spine matters even less with these section, so again, its not a big deal. The ease of storage and transportation of 4 piece rids is worth any small amount of trouble.