Is this the filly you are having the issue with? Sorry I couldn’t figure out how to screen shot. https://forum.chronofhorse.com/uploads/default/original/3X/f/5/f547668f29277cec16e050bfb22d5b03d94929ed.jpeg
Her SI conformation is not that of an FEI jumper and having back issues at 4 years old, I believe could be a result of her conformational fault in that area.
If you expect that your filly has the potential to become an FEI 1.60m jumper, I would consult professionals (not the ones who sold the horse to you) whom have trained and jumped successfully at that height, and ask them if it is fair to your horse to ask her to aim for that height with the issues she has already. They can ride her (or just look at her) and give you an idea if they think she could possibly be capable of the 1.60m level of FEI jumping.
A sore back and that SI conformation at 4 aren’t a good indication that she will have success at the upper levels.