I want to fly in area that Airmap shows as a yellow zone.  Will I need to call the airport or can I get authorization through the DJI go 4 app?  Also will the Go 4 app warn me?  

Look here: https://www.dji.com/mobile/flysafe/geo-map

If you press the question mark all the different zones will be explained. You can even choose your drone, so you get the info relevant for you

Hi there, thanks for reaching DJI. In these Zones, which appear yellow in the DJI GO map, users will be prompted with a warning and flight is limited by default. Authorization Zones may be unlocked by authorized users using a DJI verified account. You may also visit this link for more information: https://www.dji.com/flysafe/geo-map. Should you have further question, please let me know. Thank you.

DJI Natalia Posted at 11-4 12:36
Hi there, thanks for reaching DJI. In these Zones, which appear yellow in the DJI GO map, users will be prompted with a warning and flight is limited by default. Authorization Zones may be unlocked by authorized users using a DJI verified account. You may also visit this link for more information: https://www.dji.com/flysafe/geo-map. Should you have further question, please let me know. Thank you.

Thanks for the link.  I entered the location I want to fly and there is no restricted zones.  The Airmap app showed a yellow zone.

Christine96 Posted at 11-4 13:23
Thanks for the link.  I entered the location I want to fly and there is no restricted zones.  The Airmap app showed a yellow zone.

Hi, thanks for your updates. We're glad that you're not located in a restricted area. I would recommend flying your aircraft in an open area to avoid the incident. Thank you for the support. Have a safe flight ahead!