If she lands on her shins the skill is considered "incomplete", as you must land on your feet to complete the backhandspring. The deduction for incomplete element is "up to the value of the element." The backhandspring in the L4 routine is worth .6, so I would take .2 as she completed the majority of the skill. I'm sure there is an argument for taking the full .6 similar to how we do on beam with missing feet on a backwalkover/cartwheel/handstand, however, I do believe with the L4 backhandspring that getting to the shins is essentially completeing the skill...so I only take a small deduction. In addition to the "incomplete element deduction" she would also receive a .5 deduction for the fall and any additional execution deductions (bent legs, bent arms, etc.) which I'm assuming would be plentiful if she is ending up on her shins.
With a spot the deductions are: .5 for the spot + the value of the element .6 + any execution deductions. So she will lose a flat 1.1 plus bent legs, arms, lack of rebound, no stick, etc.
So, it has the potential to end up pretty much the same if the judge chooses to take the entire element deduction for incomplete element. However, at least in my area in Region 5, most judges would give the gymnast credit for completing more than 1/2 of the skill and only take .2+.5(fall)+execution.
If I haven't completely confused you, I hope I've helped.