I have a wifi only iPad 2 that I use in conjunction with a Verizon 4G mobile hotspot. That's really the only option since the 3G and 4G networks require different radios. Thus, the iPad 2 cannot be "upgraded" without a hardware modification and that ain't happenin'.
The advantage, of course, is that in areas where 4G is available, I get near desktop wired cable speeds. Likewise, I don't have to have a 3G iPad, saving me the $130 premium for that model. Further, the mobile router can be used with my laptop and my smartphone along with the iPad. (A similar approach is available if you tether a 4G smartphone to your iPad.)
The disadvantage is that I have a two year contract with Verizon (rather than the month to month 3G contract available with the iPad) and the cost is $50 per month rather than $30.