As a foldable, the Galaxy Z Flip 4 has a much more fragile display than a normal phone. This is what allows the screen to bend, but also means it's much easier to damage, and will cost a lot more to replace.
The outside of the phone is made of toughened glass, so would hold up to a drop or scratch better, but again this is an expensive phone to buy spare parts for. That's why a case is a wise idea.