Issues with opening files in Dorico 4

I’ve a cluster of issues linked with opening files in Dorico 4. I’d be grateful if anyone can offer any solutions.
(1) I can’t open two .dorico files at once in Dorico 4. If I try, I get an error sound and a message saying “Error: Invalid parameter”. It’s not the files at fault, for I can open them individually. This was never a problem in any previous version. Any idea why?
(2) I’m wanting to compare two similar versions of the same vocal score. Does Dorico have a built-in parameter for comparing scores?
(3) When I open a file, I get a message saying “This file was created in Dorico 3. Do you want to update?” This query always appears in some faraway window, not on the open screen. Is there any way of getting old files to auto update when I open them, without this query every time?
(4) This one is for the Dorico team. Why is it that when I have a single project open and I click “Close Project”, then the whole Dorico programme closes. I want to keep it open for the next score. But no luck. I have to close and open it every time. Can’t you make it to stay open when a project is closed? Is this meant to happen?

This is standard operating procedure in my Windows version of Dorico Pro 4.1.10 and has been since any version I can recall. The Hub returns to prompt for the next file. Is there any possibility that you checked an option not to show the hub when you close a file?

Thanks @Derrek . That partly answers question 4. Tho I would have preferred Dorico to stay open with a blank screen.
Has anyone got any answers to questions 1 to 3?

When opening multiple projects at the same time, you can run into problems if the file names (or the complete path to the file, including all of the names of the nest of folders on your hard disk where the file is located) contain certain characters, for example commas. This is due to the way the arguments are parsed by the software to try to work out what files to open. I imagine one or more of the files you’re trying to open, or the folders in which they are contained, contain troublesome characters like commas.

Dorico does not have any functionality for comparing two versions of the same project.

You can disable the warning about the project having been created in an earlier version on the General page of Preferences.

Thanks @dspreadbury . Would the “troublesome characters” include apostrophes? Or hyphens? Both filenames contain both of these.

Hyphens shouldn’t cause problems, but apostrophes conceivably could.

Thank you. It would still be nice if we didn’t have to re-open Dorico every time we finish one file and open another.

Make sure that Dorico is set to show the Hub after you close the last project, and it will remain running.