Actually it is the SRS not CPF which is more similar to the 401K. The SRS is tax deferred, just like the 401K, but it's better in that only 50% of the withdrawals are subject to tax. Also, both are optional i.e., supplementary. Also, early withdrawals are permitted but you suffer a penalty.

Although its hard to compare them, CPF is like Social Security Tax in the US. One is forced to contribute to a limit and then you get a fixed payment for life, from when you are in your 60s (in Singapore, you have to opt for CPF Life to get lifetime payments)