Using rule of 72, we can find the approximate IRR to double an investment for a given # of years.
$100M growing to $400M equity in year 6 is similar to $100M doubling to $200M equity in year 3, and then doubling again to $400M in year 6.
So by finding out what IRR is for $100M equity doubling to $200M equity by year 3, we essentially find out the IRR is for $100M growing to $400M by year 6; they are the same.
Here is the rule of 72 formula.
IRR = 72 / # of years to double
IRR = 72 / 3
IRR = 24%
Therefore, the approximate IRR is 24%.