hey paul,
To be honestly, after you upgraded your Easyblog from 3.x to 5.x, actually that was not possible to downgrade to use Easyblog 3.x version is because between the 3.x and 5.x version, we already revamped all the code structure and table structure.
It would be best if you can try consult with your client and see is there a way to provide us with your following details as my colleague mentioned at above reply so that we can see what we do here?
1. Your client username and password
2. Provide us these 4mb image which your client trying to upload but fail.
3. How your client upload the image from the composer? Is it through the Easyblog media manager or from the image block? (You can refer on my attached screenshot below.)