Are you missing a step? Not sure, never done it before with Verizon so I am not exactly sure what steps are involved. With that said, we can still isolate the problem.
1.If you change the DMZ to your computer and hook that computer direct to the verizon modem/router, does it work then? If not, stop right there and work on that first. If it does
2. If you remove the IP filter and allow any to connect, does that work? Try tracing the IP using Visualware, or better yet, from the office, and see where it stops.
3. If you put the filter back on, can you ping even if you can't access?
That should get me started. We need to isolate between the OS,4500, Verizon router, or ISP itself.