I have spliced billets together that had much different ring thicknesses from completely different osage trees and it worked fine. However, the resulting bow limbs were slightly different thicknesses. As long as the tiller is the same on each limb, it doesn' t matter. You may be able to trade a stave longer than 52" to somebody for a pair of billets. But, to answer your question...yes, you can lay out a 4" Z-splice on each end of your staves, glue them together, then cut the excess off. Put the splice in the handle.
George