请问CPF top up SA 的问题

楼主的问题语焉不详。你问的是买的保单是否可以减免个人所得税吧？答案是不可以。
根据 IRAS 的规定，只有 CPF 全年 contribution （各种方式的总和）不超过 5K 的，才能用人寿保单减免个税。
You are not eligible for Life Insurance Relief for YA 2019 if your total compulsory employee CPF contribution, self-employed Medisave/Voluntary CPF contribution and voluntary cash contribution to your Medisave account in 2018 was $5,000 or more.
所以人寿保单减税，SP/EP holder 最能充分利用，对 PR/SC 的作用往往就很有限了。

这么说，为了减税，CPF填补4999，税务方面既可以扣人寿保险又可以扣公积金填补金额了？

一共5千，如果CPF交了4999，那人寿保险只能扣1块而已

理论上没错，如果 CPF 不够 5K，人寿保险的确可以减税不过我估计具体数额应该不是层主的期望。你用人寿保单只能减免 5K 和 CPF 之间的差额，按层主的例子，也就是 1 块钱。
If your total compulsory employee CPF contribution, self-employed Medisave/Voluntary CPF contribution and voluntary cash contribution to your Medisave account was less than $5,000 in 2018, you may claim the lower of:
the difference between $5,000 and your CPF contribution; or
up to 7% of the insured value of your own/your wife's life or the amount of insurance premiums paid.