I don't have 4G in my area yet so I have it disabled in the device settings. But I don't see a 4G only setting in juicedefender. So I assume the auto toggle for carrier data would toggle both 3G and 4G data if they are both enabled in device settings.
I very much doubt that Juicedefender toggles 4G on the Epic. Wimax 4G is Sprint-specific and the 4G radio function is separate. And even when I use the "Data network mode" prompt that Epic Froyo pops up after I hit the Power button, it does not disable 4G. For that I just use the Notifications bar.
BTW, in my experience 4G uses less power than 3G when the 4G signal is strong, and 4G is used in lieu of 3G. But when the 4G signal is marginal, it is better just to disable it so the phone is not constantly trying to connect to 4G, then failing over to 3G. The best power performance of all is Wi-Fi, which is why I keep its Advanced sleep setting at Never. Otherwise, when Wi-Fi sleeps the phone falls back to 3G search for towers to get background data, which is much more power-intensive.
Overall, Juicedefender and Tasker are inherently unable to manage Wi-Fi or cell radios as well as a well-trained user can do manually. They just do not have the basic environment data (Where am I? What networks are available?) for free. And it's not really hard to form good habits. I know when I want to use Wi-Fi and when I want to use 4G. So it is no big deal to change those settings, along with Bluetooth, from the Notifications bar. Typically I do all that at once when I enter or leave my home or my car, in a few seconds.
The only advantage I can see in Juicedefender would come from its ability to disable data services for long windows of time, such as 15 minutes. But I
want instant email notification, which I consider a major benefit of carrying a smartphone. The only other background services I use are a minimal weather widget and a battery meter widget. (The latter would not be necessary if the Epic's own meter icon told the truth.)