Evolutionary conservativeness
Even when you can live without Histone H1, its a widely distributed protein which you find in basically all living cells (except for some yeasts if I remember correct). This points to the fact that it must be available for quite a while. The same is true for the catalytic subunit of the DNA polymerase. Somatropin is still pretty important (and can be found in a lot of organisms), prolamines are quite specific to certain cereals. I would group it 2-3-1-4.