Custom Dimensions and Metrics not accessible through Google Analytics Reporting API v4 Python client

Question

I am having trouble accessing custom dimensions available through GUI using their API names. Is there any way to find the actual api name of all available custom dimensions and metrics. Currently I am using name of the format ga:dimension<indexidfromGUI> like ga:dimension4 but this returns a unknown dimension error.

HttpError: <HttpError 400 when requesting https://analyticsreporting.googleapis.com/v4/reports:batchGet?alt=json returned "Unknown dimension(s): ga:dimension11

Here is my report body:

    body={
        'reportRequests': [
            {
                'viewId': VIEW_ID,
                'dateRanges': [{'startDate': '1daysAgo', 'endDate': '1daysAgo'}],
                'metrics': [
                    {'expression': 'ga:sessions'},
                ],
                'dimensions': [
                    {'name': 'ga:date'},
                    {'name': 'ga:dimension4'},
                    #{'name': 'ga:dimension11'},
                    #{'name': 'ga:dimension13'},
                    #{'name': 'ga:dimension14'},
                ],
                "dimensionFilterClauses": [
                {
                    "filters": [
                        {
                            "dimensionName": "ga:eventAction",
                            "operator": "PARTIAL",
                            "expressions": ["ScreenView"]
                        }
                    ]
                }
                ],
            }]
    }
).execute()

If the format for api dim name is wrong, how do i find it out? It is not available in Analytics GUI as far as I know. Is there a documentation for this? I am aware of https://developers.google.com/analytics/devguides/reporting/core/dimsmets. But this doesnt include custom dimensions.