Any third party proxy server that you can find on the Internet, which uses other than port 80, should work. As long as they let you waste 4 Gb of bandwidth.
If you want to stick to using an NTL proxy, here is a solution. You can use one of the still remaining, old proxies of the form
cacheX-lang.server.ntli.net:8080 (where X is 1, 2 or 3).These return the correct file size. Unfortunately, these proxies do not support download resuming, so if you are almost at 4 gigs and the download disconnects, you can start all over again.
An avenue I tried is to start the download with one of the cacheX-lang proxies to get the right size, and download a few megabytes. I then changed back to one of the standard NTL proxies and resumed the download. The download does resume, but it stops at 13.7 Mb.
So it looks like the only solution at the moment is to either use a third party proxy, or use one of the cacheX-lang proxies for the entire download, and pray that you don't get disconnected.
EDIT: I can also confirm that nffc's solution (proxy IP 69.19.14.10 port 3128) works wonderfully. Even though it's a proxy in USA, I am getting full download speed at the maximum possible with my 2 Mbps connection.