Google Analytics Python API "Invalid JSON payload received. Unknown name "segment" at 'report_requests[0]': Cannot find field."

I followed the Google Analytics (GA) Hello Analytics tutorial and used the query explorer to call the GA API via Python and this worked great until I tried to include a segment in my query. It seems that the query explorer's use of segments (or my understanding of how to use this) is not correct. Here is my get_report function:

def get_report(analytics, segment):
if segment == "all":
    bdy = {
        'reportRequests': [
            {
                'viewId': VIEW_ID,
                'dateRanges': [{'startDate': '2021-01-19', 'endDate': 'yesterday'}],
                'metrics': [{'expression': 'ga:uniqueEvents'}],
                'dimensions': [{'name': 'ga:eventLabel'}]
            }]
    }
elif segment == "internal":
    bdy = {
        'reportRequests': [
            {
                'viewId': VIEW_ID,
                'dateRanges': [{'startDate': '2021-01-19', 'endDate': 'yesterday'}],
                'metrics': [{'expression': 'ga:uniqueEvents'}],
                'dimensions': [{'name': 'ga:eventLabel'}],
                "segment": "gaid::aqYF4QvFTX6JhLKfs0R84A"
            }]
    }

return analytics.reports().batchGet(body=bdy).execute()

The first if block works perfectly fine but when I try to run the second I get this error:

HttpError: <HttpError 400 when requesting https://analyticsreporting.googleapis.com/v4/reports:batchGet?alt=json returned "Invalid JSON payload received. Unknown name "segment" at 'report_requests[0]': Cannot find field.". Details: "[{'@type': 'type.googleapis.com/google.rpc.BadRequest', 'fieldViolations': [{'field': 'report_requests[0]', 'description': 'Invalid JSON payload received. Unknown name "ga:segment" at 'report_requests[0]': Cannot find field.'}]}]">

It seems like the "segment" key in the second body dictionary is not correct, but I can't find what it should be. Does anyone know how to correct this?

In case this is helpful, this is what the query explorer suggested I use:

"query": {
"start-date": "2021-01-19",
"end-date": "yesterday",
"ids": "ga:xxxxxxxx",
"dimensions": "ga:eventCategory,ga:eventLabel",
"metrics": [
  "ga:uniqueEvents"
],
"segment": "gaid::aqYF4QvFTX6JhLKfs0R84A",
"start-index": 1,
"max-results": 1000

}

I think the query explorer might use v3 of the API, while I'm using v4....

Thanks in advance!