Separeate network or Obri's switch

I am trying to setup a Guest WiFi on the Orbi (SXR80).  We are running two seperate routers (one for our main & one for our Customer).  I would like to combine these on the Orbi onboard switch.  Currently the Orbi is setup as an Acces Point as it just get it information from our Barracuda Router/Firewall.  This would be our main network that contains all our Servers/computers and company devices.  This is setup as the main WiFi when clients that join this have access to our network and servers.  

I would like to setup the Guest WiFi on the Orbi using the 2nd Router which is separated from our network.  I am going to use Wireless 3 so I can setup a secure connection that requires a Passoword when connecting to the SSID.  

My question is can this router's ethernet output be plugged into Port 4 of the Orbi?  Should I set that port to VLAN 40(Guest) & Trunk?  If the Wireles 3 is set to VLAN 40 as well will this setup work?  Will port 4 be dedicated to just that router with VLAN 40 and will the clients connected to Wireless 3 get the access they need without access to the main network (main rounter)? 

Is there any chance of a network loop or errors in traffic? 

Is my thinking right here and are the switch ports on the Orbi advanced enough to perform this?