I guess I found a for me (for the moment) good enough solution.
v4-Implementation:
const collectionType = 'api::page.page';
const allPages = await strapi.entityService.findMany(collectionType, {
fields: ["*"],
filters: { },
sort: { },
populate: {
// content [is the name of the dynamic zone in page]
content: {
populate: "*"
},
}
});
In my v4-test-project I have multiple components (one with multiple string-attributes, one with an image, one with a relation…) in the dynamic zone of page.
For them the above implementation seems to work.
I hope this helps others. 
However if I want to include all fields of my relation (for example to another page with its own content dynamic zone) I need to adjust my implementation to:
...
populate: {
content: {
populate: {
// page [is the collectionType of the relation inside of the component in the dynamic zone in page]
page: {
populate: {
content: {
populate: "*"
}
}
}
}
}
}
...
but then I’m missing the fields of the components in the dynamic zone of my page collectionType.
Is there a trick how I can achieve both?
Cheers,
Olaf