Is there a way to use 5 star skills with 4 star units?

If you promote one copy to 5*, learn the weapon, and merge the 5* into your 4*+7, then your 4*+8 now has Camilla's Axe learned. I also read that you can refine the weapon from the 4* character; you would achieve the same effect by refining the weapon while it's on the 5* and then merging onto the 4*. Note that this does not mean you can refine a weapon and then inherit the refined weapon to somebody else - that doesn't work; only merges keep all the refines that you've spent stones/dew for.