By default GD4 save a project file in .gdg format which is not compatible.
You need to save the project in .json format.
Go to File → Save as
And enter myproject.json as the file name.
After you should be able to open the json in GD5 except if you have been using any native only features.
Check your project in GD4 if it works with the HTML5 platform. If it not working, it won’t work in GD5 either not even if you save it as json. GD5 is HTML5 only.