My daughter's 3G Kindle reader can access any web page on the net (for no additional charge) but the "experimental" browser is so horrible to use it's like pulling teeth. It's not touch screen or anything so you have to use the cursor keys to move the cursor around the screen to get to any links on the page. It can't download content, save pictures or play movies, etc... nothing that could use a significant amount of data.
The Kindle Fire HD has a full featured browser and the 4G will enable it to surf the full web and download whatever, including streaming movies from Amazon. The wimpy 250M/month allotment could very easily be used up in a few minutes. Hopefully there's some kind of safeguard that would prevent someone who doesn't know any better from downloading Gigs of data. It costs $30 for an additional 3G of data. I'd be willing to bet that if you used 260M in a month you would be charged the full $30 extra. Check out this article: Amazons Kindle Fire HD data plan, don't be misled.