It is good to be optimistic but I'm trying to get a handle on your numbers
You have 100k now.
Income is 130k
Taxes guess 25k
Expenses guess 35k
Savings 70k/year
Let's ignore taxes for now. What rate of return would you need to reach 750k in the next four years?
Answer: 25%.
I have a few minutes to look into this myself, so I figured I'd give it a try too.
Gross of $130k
-$6650 HSA
$123,350 ($9450 towards FICA)
-$2500 Medical benefits
-$36,000 401k x2
-$11,000 IRA x2
-$20,600 Standard deduction + personal exemption x2
---------------
$53,250 AGI
This AGI falls to more than $6000 under what I see appears to be the AGI cap for deduction of IRA contributions. Someone correct me if I'm doing this wrong, but yesterday when I asked the same question I was basically told that's right. The current phase-out start for MFJ and TIRA deductions seems to be about $60,000.
For married filing jointly that should be $7080 federal income tax. No idea what he's paying in state, but I've seen it frequently run around 20% of federal for married couples in a lot of states, so let's estimate $1416 state.
Total taxes: $7080 fed, $1416 state, $9450 FICA for a total of $17946 in taxes.
So divided up it appears to work as follows:
$130k gross:
$18k taxes
$2.5k medical insurance
$57.5k tax advantaged savings (lowball estimate of 3% 401k matching and no HSA match)
------------------
$55k left over
If they truly are hardcore saving, I'm going to estimate spending is even lower at $30k. That leaves $25k to invest after taxes for a total of $82,500 saved per year, with more than 2/3 of that amount in tax advantaged accounts. Still, even the after tax money is going to be almost entirely tax-free since they are well within the 15% tax bracket, and both dividends and long-term capital gains are taxed at 0% when you're within the 15% bracket. So for the sake of this example I am willing to say we can consider ALL of the money invested to be accruing value and dividends tax-free.
OP says he's going to have $100k when turning 24.
Age 24: $100.0k * 1.07 + $82.5k = $189.5
Age 25: $189.5k * 1.07 + $82.5k = $285.3
Age 26: $285.3k * 1.07 + $82.5k = $387.7
Age 27: $387.7k * 1.07 + $82.5k = $497.4
Age 28: $497.4k * 1.07 + $82.5k = $614.7
Age 29: $614.7k * 1.07 + $82.5k = $740.2
If we revise age 29 to be "age 28 years and 364 days" then I can see it being very plausible that he could reach around $750k while technically still being age 28. That's based on a consistent 7% ROI, which I see as overly aggressive when rated against the rate of inflation, but OTOH he's young and statistically
very likely for he and his wife to get big raises in the next 10-15 years. I see his plan as very much doable, especially if the idea is to keep working to age 35 or so.