I believe the way API gateway maps the query parameters you need to comma separate your accountIds. Aka https://my-dev-host.com/fdx/v6/accounts?accountIds=confUser_1,confUser_2,confUser_3,confUser_4&offset=1&limit=3
accountIds
https://my-dev-host.com/fdx/v6/accounts?accountIds=confUser_1,confUser_2,confUser_3,confUser_4&offset=1&limit=3