Unexpected sale of rented apartment. Options?

BUT: How should a regular termination be justified? A proper termination by the landlord is legally excluded!

1. Purchase does not terminate the lease
2a. When was the conversion into condominiums done? Does the landlord own the whole building?
2b. Termination for personal use is possible, provided the conversion was done before the start of your lease, but with relatively long notice periods
2c. Termination for personal use impossible (blocking period), if the division into condominiums was done only now
3. The property is quite old already. What about the maintenance reserves? Condition of the entire building ==> possibly a new heating system is due or the roof needs repair. People tend to get rid of an apartment or the entire building in such cases.

I currently see equity destruction lurking around every corner, which is sabotaging our plans for my parents' house.

I wouldn't necessarily buy. Depending on the price, it can cost a lot of money. The additional purchase costs are simply high. With luck, there might be a break-even point if the value increases. Renting elsewhere doesn't necessarily have to be complicated. Hire a broker to search and let a moving company do everything. That also costs money, but you aren't tied down. And 4 years is not a transition. A transition is a few months.

Since much has already been written here, I will only make one comment on the topic of capital protection.

In case you continue to live in the apartment as a tenant but want your equity to be worth as much in 4 years as it is now, I invest in a broad equity ETF that holds stocks from the largest companies operating worldwide. You can't go wrong with that. For example, the VANGUARD FTSE ALL-World.