Have you opened a support ticket?
If not, please do. I suspect one of us will need to take a look at your machine to understand what's happening. This isn't a common issue, so it is likely specific to your system in some way.

Output > Force Stage Preview, and make sure that Output > Show Stages works

 @ewan_fp Can you confirm that you did both of these and still saw no output?

Also, please try this version of your file in which I've put a output test Scene at the beginning. I used the Show-Hide Stages actor to programatically show the Stages in case that was the issue: gad-showfile-4-february-24th-v4.0.2-l-edit.izz

I just took a look at your file, and as you mention, you are using one stage through out. When I open Stage Setup, I can see that we have 'Stage 1' as required.

When I open the stage fullscreen or as a preview, I see the output as expected. 

Are you testing with a connected display or only a projector?

You mention Isadora 4.0.2, you might want to update to the latest version. There have been updates that effect output, although this issue isn't one I have seen reported. 
TROIKATRONIX : ISADORA - Get It

gad-showfile-4-february-24th-v4.0.2.izz

Please let me know if you discover anything awry.

my stages

Re-reading your original post, I think this is the key here; you are working with multiple Stages. By default, Isadora 2 had 6 Stages (if I remember correctly) and that was also the maximum number of possible Stages. By default, Isadora 3 and 4 have 1 Stage, but you can add up to 15 more by clicking the little "+" button at the bottom left of Stage Setup (keyboard shortcut is Cmd+2) as well as up to 32 Virtual Stages (Stages not assigned to displays that can be used to output Syphon, NDI, Blackmagic, or for pulling content back into your patch for some advanced video workflows).

TLDR: If you're working with multiple outputs you need to add more Stages in Stage Setup.

Here's that Stage Setup tutorial again, it might make things a bit clearer: https://youtu.be/https://youtu.be/SIB31xqsxto?si=jIXG52wbcOkKpntI?si=jIXG52wbcOkKpntI

Yes, a show file would be extremely helpful.

The Stage Setup changed completely from Isadora 2 to Isadora 3, so upgrading a file directly from Isadora 2 to Isadora 4, your Stage Setup (Cmd+2) is basically going to be a blank slate that needs to be configured.

More Stage Setup Resources:

Short video: https://youtu.be/https://youtu.be/SIB31xqsxto?si=jIXG52wbcOkKpntI?si=jIXG52wbcOkKpntI

Long Video: https://www.youtube.com/live/wX7tbFUFiwE?si=HGYCsTbTowwQdmUC

If you’re jumping straight from Isadora 2 to 4, you may want to run through the Isadora 101 video tutorial playlist made for Isadora 3, as it covers the basics (including many of the major changes between Isadora 2 and 3 like Stage Setup) and should still be accurate info for Isadora 4: https://youtube.com/playlist?list=PLqiBJCe8prQuKpxQZUwaye7chQG_gvweg&si=aMDBu_0PmoliLsgk

Hi! Welcome to the Isadora community forum, could you maybe share a base show file with us so that we can verify it on our end?

Some general advice:
1) Make sure that you have your displays setup (extend mode), and turn off Displays have separate spaces, as Isadora asks you when you first open the program

2) Go to Stage Setup > make sure that your stage is connected to a 'Display'

3) Do you have your 'content' on the 'Force Stage Preview' output? You can check this by going to Output > Force Stage Preview, and make sure that Output > Show Stages works