First, restart your computer as you have some processes running in the background that won't allow you to install Adobe Photoshop Elements 2021 at the moment (that's the reason behind the Error code 81). That should stop them running. You can check if they are still running by looking at this Adobe Knowledge Base article:
Then, go to Hämta Adobe Photoshop Elements and download a copy of Photoshop Elements 2021 from this page. The first error you received (Error code 4) was due to a "damaged" file on the disk (it probably isn't damaged and may be just a read error that occasionally happens when accessing files from a DVD-ROM).
Now go to Installera Photoshop Elements and scroll down the page to the section "Hämta och installera från Adobe-webbplatsen" and follow the instructions from there.
Hopefully everything will go OK from there. Please let us know how you get on 🙂