Generally untreated Ceylon Blue Sapphire is quite expensive. For 4.35 carat it would cost around $3k – $5k. Please note that it is difficult to identify origin of gemstone even by using gemmological instruments. So, even if someone is selling you a gemstone (blue sapphire) saying it is from Ceylon (Sri Lanka); it might not be actually from Ceylon and no one will be able to identify its origin.
Since, your budget is okay for a blue sapphire, I would suggest you to go for blue sapphire of any origin as long as it is untreated and natural. But please get it verified from gemmological lab.
I would also suggest you that, if you are buying from local gemstone shop then ask the shopkeeper to first get it verified before making the payment and always check the certificate thoroughly if the size, picture mentioned in the certificate matches with the gemstone or not.