Apple's not about moving downward on battery specs. You won't see "4G iPad, now with a 5-hr battery!" When you do see 4G on an iPad it'll be because they have a beefier battery.
No, that's for certain it will. Apple makes great products; I would just imagine you'd get even greater life out of one connected to a MiFi rather than funnelling juice into a WWAN card. That's all.
To me it comes down to:
Pro MiFi
Able to use w/ multiple devices
4G now
Cons MiFi
Contract
Extra Device to cart around