Even if they are a totally legitimate seller of Disney tickets...take a look at the site and the price list.
The only ticket they are 'upgrading' from 4 days to 6 for 'free' is the straight park ticket, no expiration, no hopping, no plusses. Big deal. The price difference from 4 to 6 days on a ticket like that isn't much.
If you expect to be able to get a park hopper with plusses, 6 days for the price of four, you're probably out of luck.
Like the other poster said...If it sounds too good to be true...