Please help — how do I lengthen *only* the back rise??

My daughter has a long torso, which poses a problem when making pants… Height-wise, she is 44" (size 5, in O&S size chart), however her waist/hip width and leg length is a 4. So, a size 4 is right, except for the rise… So, because of her long torso, it makes the top-back of pants not high enough (not covering her bits). The front, however is the *perfect*! rise -- oddly, her rise is only longer in the back (!?!) How on earth to I mash those two together?? I've tried cutting the size 4 everywhere, but the size 5 height on back crotch to waistband, and size 4 in front. But then it's really wonky to blend those two (gracefully). It creates weird sharp angles (and doesn't lay well). Oh, yes, to add to that…super flat bum!! So, no extra fabric is needed back there -- just height! (I'm nearly in tears after making her a BEAUTIFUL tailored pair of Picnic Shorts, using a size FIVE so they'd fit her back length. The back length is awesome, but the rest is awful: too high on her front waist and super balloon-y around her bum and legs.) :'(