Unexpected sale of rented apartment. Options?

Sorry, but 4 years is way too short an investment horizon for ETFs! There is a great risk that the global market goes through a crisis and the equity capital is only worth two-thirds or even less.
For ETFs, an investment horizon of at least 15 years generally applies.
The advice as it stands is reckless and dangerous!

The property is risk-free? ;) Of course, the stock market can go down, but do you seriously believe that real estate prices will continue to rise if the stock market falls dramatically? If it has 4 years, I see absolutely no problem in the current phase. But sure, it can also turn out differently! Historically, there was one time when it took 15 years to break even.

By the way, there was also a real estate crisis where prices collapsed!

And then maybe 2 years later you have the problem again..

What should it cost so much in a private purchase? 5% that’s how much a real estate agent and the move cost.

Depending on the federal state, the additional purchase costs range between 5.5-8.5% of the purchase price. That would already be at least 10k for a 65sqm apartment. Probably even more, since the OP mentioned a large open space. With a real estate agent and the move, you’re at 5k. Quite a significant difference.
And discussing "what if" is futile. What if the homeowners association enforces a heat pump and facade insulation in the year of purchase?

Buying and betting on rising prices to recover the additional purchase costs in 4 years is pure speculation.

And discussing "what if" is pointless. What if the owners' association enforces a heat pump and facade insulation in the year of purchase?

You can see it in the minutes. And you can torpedo and delay it.

But to each their own, you are very cautious, 10 years ago no one thought that even in Hinter Tupfingen property prices would triple.

And you also forget that you will pay rent for another 4 years.

I would buy it.

Don't be unsettled or intimidated by the risk. If the money is in the bank in the checking account, then after 4 years you also have a loss, you don't see it but it is there because you can buy fewer goods with the money. If you invest in the property, that's good, and if you take the stock ETF, that's also good. Both are associated with risk, of course, but nowadays it can't be done any other way. My God, the whole life is a risk!

The property is without risk? ;)

It is completely irrelevant whether a property also has a risk, because you advised investing the equity in an ETF until the house is built if he wants to keep renting. And that is definitely much riskier than a fixed deposit, which currently cannot offset inflation, that’s true, but at least does not suddenly crash by 20-30%. At most, one could advise investing a part in an ETF.

If he has 4 years, I see no problem at all in the current phase.

What crystal ball are you using? The Optimist Mk II?

But of course, things can turn out differently! On average historically, there was one time when it took 15 years to get back to around break-even.

And exactly for that reason, a 15-year investment horizon is recommended.

Would the apartment be no good for your parents? Otherwise, they might possibly buy it and you pay them rent until you swap in a few years? Or do they have other plans?