If you tap your 401k before you're 60, you will face major tax penalties. So, that's at least 20 years you have to manage without your 401k funds if you attempt to retire at 40.
You will still be making money each year from your 401k, though.
Thus, for the math to work you need to verify that you have (or earn via interest/other investing) twenty years of expenses via funds outside of your 401k.
The earlier comment also omitted the backdoor Roth, which can get a married couple another 11k post-tax into a Roth, which can then grow and be used tax free.
An interesting exemption (basically, you set up a structured, regular payout of your 401k over the remaining life expectancy and don't touch it). I still don't see this being realistic in almost any case, but it's a good option to keep in mind.
That said, "$3916 a month from when you were 18 until 40" is simply not realistic for most people. How about college? You're not going to get a $100k+ a year job without either college or years of experience. Where do you find that perfect mate capable of immediately working their own $100k salary and contributing almost $2k a month to retirement at 18?
Personally, I make a really good salary (it's worth noting that it took until I was nearly 40 to even get to this point), have a working spouse, live in a relatively inexpensive part of the country, and I still could not put $4k into an 401k or roth IRA every month. Owning a house and reliable cars, having dependents... it's not realistic; it leaves nothing for wiggle room.
I'm also going to come out and say it: working yourself to the bone, skimping and saving is no way to live some of the most active years of your life.
"I'm also going to come out and say it: working yourself to the bone, skimping and saving is no way to live some of the most active years of your life."
Exactly the response to all of these "retire young" articles. They neglect to say that you are doing it at the expense of not taking advantage of a healthy young body to adventure and enjoy life while you can...you may get hit by a bus tomorrow and then what a freaking waste of time all the penny pinching was!
Thus, for the math to work you need to verify that you have (or earn via interest/other investing) twenty years of expenses via funds outside of your 401k.