Licensing Mathcad Prime Upgrade 4 to 5

In general: not.

It's a new version of the software. You pay again.

Unless.... look at the botom of this page https://www.ptc.com/en/products/mathcad/new-release

It suggests that some people get it for free. Or do they get just the newsletter for free?

You should be able to install Prime 5 in Express (free, no charge) mode on any computer, including the one loaded with Prime 4.

They should both work, next to each other, even simultaneously. However you may find that translating Mathcad (15 and below) sheets to Prime will no longer work, and there may be other caveats....

I'd suggest you install Prime 5 on another computer and use the (full functional) trial period of 30 days to investigate if Prime 5 is worth spending money on.

Success!!
Luc