Would a seperated 90cmx45cmx45cm tank be suitable for 2 pairs of angels to breed in? That would give them a 45cm cube.

Id say yes

Cool, Could the seperator be see through? Or would i have to make it so the pai couldnt see each other?

i would recommend the pairs not being able to see each other, they get very firey when spawning.

It is hard to know and may depend on your pairs. That is about the territory they will defend so may help them be better parents if you are leaving the eggs. I have not done it and always removed the eggs as I wanted to rear angels rather than watch them eat the fry. They are big fish and I think would be better in a 600 x 300 x x300mm tank for each pair.

IMO, if the pairs can see one another, the effect will be similar to having dither fish that bring out the parenting defense mechanism, and better care for their brood.

In one case of mine, a pair of apistogramma paduros, with no dithers or having viewable fish, eat all their eggs before they hatch.

Yet a pair of A. nanay melgars, in a divided tank have raised one brood of 26 and are now working on another brood from a spawning of about 100 eggs.

What works for one MAY work for you.