OK, so, if you are going to be using Port 4 on both devices to connect each other, and assuming you have the VLAN's setup on the GX50 properly already:
1. On the GX50, set the port mode to "trunk", you can keep VLAN1 as native.
2. On the XS716T setup the VLAN's that you have defined already on the GX50. This is covered on page 130 of your manual:
3. On the XS716T you'll be looking to setup the equivalent of the port mode to "trunk", also leaving VLAN1 as native. You may have to just setup the port with VLAN1 as untagged and tag your other VLANs, I don't use much Netgear equipment and some manufacturers use different language.
4. Assuming your Orbi isn't setup to handle VLAN traffic, you want to setup Port 5 as being an access only port with VLAN4 being untagged.
This assumes you want to have different devices connected to different VLAN's on that switch. If the only thing connected to it is going to be that Orbi, then the easiest way is to just set the port on the GX50 to an access port, assign it to VLAN4 and then only VLAN4 traffic will hit the switch.