What's the file transfer speed of Bluetooth? 5 files in 4Gb = 800Mb each. Transferring 800Mb over Bluetooth is going to be slow I would have thought. Unless it transfers them compressed and uncompresses them before use. But it obviously can't uncompress on-the-fly otherwise you wouldn't restrict it to 5 books, you'd have however many compressed versions you can fit. So if it can't do on-the-fly uncompress then it's going to take an appreciable amount of time after transfer to uncompress.
Either way I think they're counting on transferring books being an occasional activity. I think this is aimed at the casual reader who might load up a couple of books, read them and then a few days/weeks later, load up a few more. Those of us who load up a significant proportion of our entire library "just in case" probably aren't going to find this a useful device.