Paying pointless interest on 100,000€ for 1 year, being tied to the bank, and the increased notary costs for the twice registration of the land charges is not something I consider sensible. But that is just my personal opinion.
Why should the interest rate rise to 4%? Just because it briefly makes a short counter-move now?
Yes, you are absolutely right, it depends entirely on what you want to achieve. If you want security, then you should do it that way. The notary costs amount to a few hundred euros, which is not much compared to a higher interest rate.
The interest rate averaged about 5% over the last 20 years. If the market's return expectations change sustainably, it will go there again. The current low-interest phase is precisely that, a phase, and it will end at some point.
But for the loan you need security, e.g., a land charge. Either you already know the required financing structure today or you have to have the land charges adjusted next year at a cost (notary and possibly bank)...
That is correct. Security in the form of the property already exists. Further financing will be arranged when the plan for the house construction is also finalized. That is not a problem, and the notary costs really remain within limits.