Given that he says that F11 and F12 were skipped because they didn't exist on the IBM keyboards at the time and given that he listed them starting from F10, it

Given that he says that F11 and F12 were skipped because they didn't exist on the IBM keyboards at the time and given that he listed them starting from F10, it seems reasonable to assume they started assigning the keys from F10 working down, making F4 the next after F5. Also unless things were different early on, the functions you're listing are the unmodified ones, not the functions you get when holding alt.