If the expansion had been slow enough to allow full equilibrium to be maintained as the universe expanded and cooled, all that would be left would be iron-56, not hydrogen and helium.
Actually, He-4, once formed, is really hard to break (~2MeV/nucleon, or 20 billion Kelvin above the average temperature, or 1 standard deviation, as you can see from this graph), so the 1⁄4 ratio of He-4 by mass would have persisted regardless of the cooling rate. The rest would be carbon, oxygen and iron.
Yes, breaking up He-4 is very endothermic. There is a triple alpha process, which was too slow to proceed much in the big bang, which converts 3 He-4 → C-12 and is exothermic.
Actually, He-4, once formed, is really hard to break (~2MeV/nucleon, or 20 billion Kelvin above the average temperature, or 1 standard deviation, as you can see from this graph), so the 1⁄4 ratio of He-4 by mass would have persisted regardless of the cooling rate. The rest would be carbon, oxygen and iron.
Yes, breaking up He-4 is very endothermic. There is a triple alpha process, which was too slow to proceed much in the big bang, which converts 3 He-4 → C-12 and is exothermic.