As you can see on https://twitter.com/powtac/status/277190821479063552, strings with four letters starting with an $ are marked as a Tag on twitter. But what kind of tags are those $TAGS? 
Why are strings on Twitter starting with $ marked as tags?
twitter
That's part of the stock exchange search feature. Or cashtags. When you include a dollar sign followed by the stock ID it will take you to a search page that shows you related Tweets about the company for that ticker symbol.Now you can click on ticker symbols like $GE on http://twitter.com  to see search results about stocks and companies@twitterSince there are a lot of the stock exchange identifiers with four characters, that's why you're seeing it appear auto-linked in your Tweets. Normal currency and dollar amounts don't contain letters like that in that position and is also why that convention was adopted by Twitter.