I'm not in the US so can't comment on availability - though I'd be amazed if the 4G iPads weren't available from retailers other than Apple.
Re. 4G/LTE; iPads are available in two versions (I'm ignoring colour and capacity here) - WiFi-only and WiFi with cellular capabilities.
WiFi-only versions connect to the internet by WiFi (like you probably have at home, or through WiFi hotspots at e.g. Starbucks, McDonalds etc…) but when there's no WiFi, you can't get online. With a WiFi+Cellular iPad that's set up with a service provider/carrier (e.g. AT&T, Verizon etc…) you can get online when you're out of range of WiFi, though you are charged extra for the data you use.
4G/LTE is the latest, fastest version of this cellular network, though all the other cellular networks (3G, 2G) remain available if required, so if a cellular-capable iPad can't get a connection to a 4G network, it drops down to 3G - you're still connected, but at a slower speed.