@wally - Thanks for posting your question on the forum!
You can set any of the ethernet or gig-ethernet ports on the 4430 to be a trunk to a switchport already in trunk mode. You are on the right track with setting the encapsulation to 802.1q on one of the ports. There are two things you will want to keep in mind when doing this:
1. AOS only supports 802.1q encapsulation when set to trunk. If you are using a NetVanta switch, this isn't a problem, but you will want to verify this is the encapsuation on any non-Adtran switches.
2. Unlike on NetVanta switches, where the native VLAN is set to VLAN 1, you will need to set a native VLAN once you enable 802.1q encapsulation on an ethernet or gig-ethernet interface.
Please do not hesitate to let us know if you have any further questions.
I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post to unmark it and select another in its place with the applicable buttons. If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.