Why is these unit so cheap>

rent stabilized tenant in 4F according to previous listing at 450k, roughly 70% discount over 3F, worth it for an investor?

For someone with disposable income, it could very well be worth the risk. If you aren't super wealthy and the tenant decides to stay forever, you've made a bad decision.

Yup, I wish I had that kind of money to gamble with, or maybe I wish I was the tenant in 5R paying $ 1,240 per month

I just started working on an apartment in the building and a lot can be done with the empty spaces but they need a ton of work which will be very expensive