I feel like my question isn't being understood here.

I understand the difference between the two and if the sites were for example English and Spanish I would get which to use, but the hreflang tag sub-divides languages into individual regional variants, in my case en-GB, en-IE, en-AU and en-NZ.

So, my question is which to use when the "base" language is the same, so in all essence, the article is basically identical, with the odd regional spelling variation, e.g. colour and color, tossed in?

So, you are saying to use both which seems to be contrary to all the articles I've read about this, which seem to suggest you should only use one or the other, but never really talk about this particular scenario.

I have four English sites for four different countries, UK, Ireland, Australia and New Zealand and I want to share some content between the sites. On the pages that share the content, which is essentially exactly the same on all 4 sites, do I use the hreflang tags like:

or do I add a canonical tag to the other three pointing to the "origin", which would be the UK site?

I believe it is best practice to use one or the other, but I'm not sure which make sense in this situation.