I'd have to imagine that 355kb is the compressed size, but that character count (assuming one byte characters, not UTF8) works out to be 3.938 megabytes.
That's a lot of code. Even more when you consider it's been minified.
It's 1.34mb on disk uncompressed but still minified. The copy Twitter is hosting right now is 1409121 characters minified but uncompressed. I have to assume the 4mb mentioned is output from some middle step (closure compiler output?) I'm not arguing the merits of that filesize, just that it's definitely not 4mb.
Link to it on Twitter's servers (I uploaded to dropbox just in case the file expires or is changed): http://a0.twimg.com/c/phoenix/en/bundle/t1-more.5d72926b7850...
I don't feel like digging through minified javascript though, so I have no clue what the discrepency is.
At a quick glance you can definitely see some differences between the different scripts we were served. For instance, in the one you were served there are some unminified portions that include the license notice for some of the code (easyXDM). In the script served to me there are no unminified portions, with everything being served on a single line.