The wife is looking to add to her small collection of rods. I found the perfect blank to build it on, but it's too long at 6'10"  Ideally, for her, I need to shorten it 4in.  What is the best way to do it? I'm concerned that I'll mess with the integrity of the blank if I just wack of the 4in. with a hack saw. Or should I leave it as is and tell her that it (the extra length) isn't that much of an adjustment and not mess with shortening it?

4" off the butt won't matter too much, but I'd bet she could get used to using a 6'10" rod without any problem. Getting the handle length right for her will have more effect than the eventual length of the rod as it will affect the handling and balance.

I use a Dremel. Chuck the blank up in my power wrapper, and get it spinning about 200rpm. A hard cutoff wheel in the dremel, spinning very fast. Use a very light touch, and the dremel will cut it off neatly and cleanly. 

ONLY the butt, not the tip.  She and you will not like it if you cut off the tip.

Put two wraps of masking tape around the blank where you are going to cut it. A hacksaw with a fresh blade will do the trick. Use light pressure, and start the cut only using the blade in reverse at first until you get a good line for the blade to follow. Let the blade do the cutting. The tape would prevent the graphite from splintering during the cut. This is an old trick my father used for years when he used to build. 

Both processes mentioned above will work, but the most reliable one is the Dremel.  It is so easy to snag a fiber on these blanks that going with the system which operates at very high speed with very low feed rate will be the best chance of getting a clean cut.  I've used both, and highly recommend the Dremel.

It's the thick end of the blank and it's going to be covered in some sort of handle and epoxy, it really doesn't matter too much how you cut it off as long as you don't actually crush the blank, it's not going to be a weak point!

Remember to use a good dust mask with power tools, graphite dust is nasty stuff to breathe in.

Second the Dremel.  I just hold the blank in my hand and use light pressure with a cutoff wheel.  It doesn't matter if it's not square.