Why does StashAway assume that during financial freedom years, all assets will only grow at 4% p.a?

During your financial freedom years, you should have a lower risk appetite. A 4% expected return is a conservative estimate for a low-medium risk, globally-diversified portfolio. Therefore, it is used for asset**  projection during your financial freedom years. 

** Asset excluding CPF accounts. CPF accounts will be projected at the prevailing interest rates.