From what I know, "4G data (3G is good, but not enough for streaming video), even on a limited plan such as 2GB a month" wouldn't be doable as free, even in the context of an 80/year Prime subscription.
"4G" service with even a miserly cap like 2G (and you'd blow through that very fast watching movies) retails for around 360/year, maybe there's a service with a 240/year plan that consumers can buy, but I don't know of one.
The big carriers are unlikely to give Amazon too much of a price break if Amazon itself wants to manage the contracts via Prime - the carriers want the individual subscribers, it's much better in their quarterly statements to say "we signed up another million retail customers" than it is to say "we signed up Amazon and gave them a volume discount."
So, if Prime costs 80, but Prime plus Fire4G costs 300 a year - is that still appealing?
(Since explaining to most consumers why their access to the faster network only works downtown but not at home, any of these plans are going to have to have both an EVDO and a secondary data connection available. Especially for Amazon, which has a very high bar to meet with customer expectations around ubiquitous network availability. I list the cell data feeds in that order because frankly, EVDO is more available in more areas from almost all the carriers right now than any of the 3.1 G technologies are.)
