I understand that if one does get asked to pay a tax bill for 4 clear years, so that’s 5 in practice, then it is unenforceable. However, if say the town hall sent a letter, albeit to an address which proved not to be correct, then they have started proceedings and in that case the four years does not apply.
As you say there is a different position re the asset declarations, but I seem to remember the wording was rather 'subtle’ so as to put that matter outside the 'four year rule.' Something to effect that the undeclared asset would be considered as an asset acquired in the year that it is discovered. I am sure Kathyslad will be able to clarify that point.