It really depends on your application. Access services (e.g. browsing and searching in a product catalog) scale horizontally without any limitations. For this

It really depends on your application. Access services (e.g. browsing and searching in a product catalog) scale horizontally without any limitations. For this type of applications, 4 huge servers are more expensive, less reliable, and not significantly easier. On the other hand services around scarce resources (e.g. a last-minute travel booking application) are indeed very hard to scale horizontally, so in such cases the parent's advice is sound.