Yes, it's exactly as if you purchase a 4G/LTE device (phone or hotspot). Since it is a hotspot device, however, there is no "voice" plan, only a data plan (although the device does have a phone number.) A "3G only" device such as an iPad or a 3G only phone will work fine with a 4G hotspot. It simply views the 4G hotspot as a wifi access point. I use my wifi only iPad with my 4G/LTE hotspot whenever I'm away from my home network. An iPad 3G would operate the same way. Simply use the wifi and don't activate the AT&T 3G data plan.
The confusion here may result from the term "3G only device." This phrase refers to the capabilities of the cellular radio in the device (i.e. iPad), not to the entire device. As long as an iPad can get a wifi signal, it doesn't matter whether the device providing that signal has a 3G capable radio, a 4G radio, or a wired connection. It's just a modem on one side and a wifi broadcaster on the other.
So, the bottom line is that you can assure yourself 4G access with your iPad (even if its technically a 3G model) by simply using a mobile hotspot. But also keep in mind that the cost of this option is significantly more than an iPad 3G data plan unless you use more than 4Gigs per month. That's the break even point (assuming you use your cell access at least to some extent every month.) Beyond that point, the 4G hotspot (or tethering to a 4G phone) is actually less expensive than the 3G iPad. Confused yet?
