higher education (whether Higher or A level or degree or something else) rests on the basis of a certain level of attainment, and will not have the time to fill the gaps
I think this is true for maybe maths, science and MFL. However for English & humanities I think that the skills seem to be more important than actual content. So it will matter less if someone didn't cover Lord of the Flies thoroughly.
Schools with good online teaching (Sutton Trust said just under a third, higher in the private sector) will see higher pupil attainment, simply as an outcome of less disruption and lost teaching hours. That'll probably mean higher results as well
I think there is no getting away from that. However I think 'picking best 3 out of 4' might well help reduce this a bit. If someone for whatever reason failed to learn well this term, but usually does then you might expect they will be weaker in stuff taught this term. Allowing their answers to e.g. Lord of the Flies to be discarded would reduce the disadvantage a bit.
I do think however a decision will need to be made by October half term at the very latest (preferably by early Sept when they see whether schools are back properly). Otherwise schools will be rushing to cover everything whereas they might be able to properly focus on new content again.