Without 4, the sequence would be 2, 3, 5. Then the next value would be 9 by fibonacci. But that's bigger than the 5 we get from adding up all the unallocated pieces (2 and 3).

We could use just 2, 3, 4 as a seed, but we can't use the fibonacci formula before the fourth element is added. Try some different seeds for yourself, it's trickier than you'd think.