Ask HN: Need help validating offer from a startup

I have an offer to join a startup as employee #41 (#16 in my country) that received ~10M $ funding from a VC besides smaller investment from a few well established companies. They have offices in 3 countries and are into core engineering work. On the conservative side, they expect a 9x increase in value in 4 years. On the upside, it's 41x. Even if successful, I don't see the company lasting more than 5 years after which I expect the product to get commoditized and margins to drop at which point someone may pick it up. I have not been in the job market for 10 years and am not sure how the startup scene works.

1) Could anyone provide me an insight into how to validate their estimation of 9x and 41x? What information could I ask to substantiate this projection?

2) I have been offered the same salary as I earn now and equity value worth 1/3rd of my salary that will vest in 4 years. I can't help but feel this is less. Any thoughts on the offer and how I could negotiate better?

Thanks!