I speculate that the interest rate will rise, and therefore I want to secure the interest rate of the [Bauspardarlehen]. And if the interest rate does not rise, I do not have to take the [Bauspardarlehen] and can continue to finance through a bank with possibly lower interest rates. At least that was my plan.
This way, I would definitely have to refinance with the risk that the interest rate might be at 4% with the regular loan.