Yes, it is possible to get the JSON plan for a speculative plan from the Terraform Cloud API, and using the endpoints you linked above. A 404 in this case is likely due to the request being unauthorized; is the token you’re providing a valid one, and one that has read access to the workspace?
I want to do the same thing but I want to have the human readable output from the speculative plan JSON. So is it possible to convert the JSON plan to human readable output?
Terraform CLI generates both the human-readable output and the JSON output from Terraform Core’s internal representation of a plan. There isn’t currently any way to translate from JSON output to human-readable output; instead you’d need access to the saved plan file that Terraform Cloud/Enterprise generated the JSON from, which is kept privately within the Terraform Cloud data store and not accessible for outside retrieval.