CPF's 4.0% "floor" interest rate for RA/SA/MA (and bonus interest rate) is (are) not strictly guaranteed. But rates have been stable in practice, including through financial crises, ultra low market interest rate periods, and a global pandemic.
It's hard to see how Singapore's well-known aging population will have much impact on CPF's interest rate schedule. The long-term ability to pay 4.0+% interest will really depend on how well GIC does over long periods of time. Although a substantial fraction of GIC's investment portfolio is centered in Singapore and thus potentially indirectly subject to unique demographic factors in Singapore, GIC invests globally. Moreover, having greater numbers of elderly Singaporeans isn't as much of a challenge if the country is open to immigration, attractive to immigrants, and offers viable pathways to citizenship.
Also, in a hypothetical future world when CPF interest rates must drop they don't necessarily have to drop for RA/SA/MA. They could drop only for OA, for example. There are a variety of ways to manage interest costs. And the government has done so already in several different ways.
In short, let's root for GIC's success.