Why would they wait for 4G? They can still make it connect to 3G as well...
They can, but it still increases the cost of production to do the dual band radio. Apple is known for bracketing their upgrades, basically not putting all of their upgrades out there at once. If they did a major change of appearance, new processor, different screen AND LTE, that would be much more than they normally do. You build in growth, not throw everything out there at once. Plus, making that many major changes at once leaves the door open for many more potential problems with the product to be refined. Just production and marketing strategies.
Like I said, I could be wrong, but I would be surprised. It would be smarter to bracket improvements so they can have releases over the next two years. Really most of the better LTE phones won't be hitting the market until last part of this year, so that would put them in line to compete with them next year.