4G LTE hasn't been discussed as much as other features, but it makes sense, given that the third-generation iPad has 4G capabilities. That, and the fact that Apple is facing increased competition from other handset manufacturers. It has to jump on LTE if it wants to keep up with rivals such as HTC and Samsung. Also, on Friday the Wall Street Journal reported that the new iPhone would support LTE, citing "people familiar with the matter."
Probability: I'd put my money on 4G. Apple's smart phone is overdue for the update.