Hi,
Some solutions are as simple as discovering the problem.
After comparing it with another website, it turned out to be related to the order of the plugins.
If others encounter this problem, here is the solution:
It would have saved me a lot of time.
It is necessary to order the plugins 'SP Page Builder - Load Modules' and 'Content - SP Page Builder' before the plugin 'Content - Fields' in 'System' > 'Manage' > 'Plugins'.
In my website I do not need the Joomla Custom module with 'Prepare Content' or the files in the ZIP file.
Just like before under Joomla 4, the shortcode {field 1} works in a 'Text Block'.
Best regards,
Jeroen