Hi! Wanna ask you think is it possible to retire at 50, with 400k in bank and a hdb fully paid up. thanks!?

Yes, it is possible but you will need to make a few assumptions:
1) that your are able to rent out your flat - room or entire flat;
2) that you are willing to make some drastic changes - live with tenants or in a rented room/ live in a country with a lower cost of living.

My best case scenario is actually for you to rent out the entire flat - say for about S$2500/mth and to use that money to finance your life in another country like Malaysia or Thailand where the Singdollar will have greater purchasing power.

With S$400k in savings, you would now be better prepared for any exigencies like if your flat is not able to be rented out - e.g. during current Covid19 climate as well as health emergencies.

I know of a Singaporean who rented out his entire flat for about S$2500 and rented a seafront condominium in Penang for about S$1500 with the remainder of the S$1000 being more than sufficient for his lifestyle there.

Of course it is!

It highly dependent on

1) The job or business you start

Assuming you start at 30, 400k by 50 is about 1.6k a month. And that is not even taking into account investments.

Assuming you save 50% of your income, then you'll need to be earning at least 3.2k for the 20 years.

I think it's possible.

2) The HDB that you buy ;)

Of course if your HDB is like $1,000,000 then it will be harder la. But if you are willing to settle for one of those $300,000 HDBs (I heard Tengah have), I think your goal is well within reach.