My new Paperwhites have 4G but when I've walked away from my router and accessed Wikipedia it will either put a 3G or 4G symbol on the top of the screen. The vast majority of times it's 3G.
As for downloading books on 3G I've done that from time to time. It takes a few seconds longer than with Wifi but it isn't terribly slow. I haven't done it enough to encounter any data caps. I doubt I've ever downloaded more than one book at a time on 3G. Probably something like one book every couple of years.
Barry