I’d take that €4B number with a grain of salt: Had the EU ordered fewer the price per vaccine would’ve been higher which is perfectly sensible not just because of economies of scale but also because a large chunk of the cost was one-off, not per-dose, costs. Development, testing etc.
As to donating the doses: It’s probably complicated, e.g. the BioNTech stuff requires sub-zero cooling which isn’t exactly easy to ensure when you’re a developing country.