Federalism would only work if all four nations had an equal voice in the federal parliament.
This would disadvantage England by dint of their larger population.
Yet anything less would deny the individual nations an equal voice in a federation.
People have a voice - not nations, which are simply constructs of people.
So any approach that would give an equal say to, let's say Wales with approx. 3 million people, compared to England with approx. 56 million people would be completely undemocratic.
To suggest that 'nations' should be given an equal say is an argument that the voice of an individual who happens to live in a nation with a small population (usually completely by chance of birth) is somehow more important than the voice of an individual who happens to live in a nation with a larger population (usually completely by chance of birth).
I have never accepted this notion that the 'voice' of Scotland isn't heard because of England - the reality is that the views of 5 million people cannot take precedence over the views of 56 million - that's called democracy, chum. And it is exactly the same for any 5 million people, regardless of whether they call themselves a nation or not. The 9 million people in London will never be able to shout louder democratically than the 58 million who don't live in London.
And, of course, independence doesn't alter this - were Scotland to become independent, you'd get exactly same issue - the views of the tiny population of the Highlands and Islands never being able to shout louder than the central belt.