Sprint caps their 3G data plans at 2 GB a month, but 4G connectivity is unlimited and way faster too. Even at a low 20% signal strength I can watch a streaming Netflix movie with no pauses or frame drops, and without having to worry about exceeding the 2 GB limit that Sprint places on 3G connectivity.
The trick is to ensure that you're only connecting to the 4G network, which is one of the settings that can be easily configured on the Sprint Overdrive.
Here's how to force your Sprint Overdrive to only connect at 4G speed:
The admin console for the Sprint Overdrive 3G/4G is a web page located at http://192.168.0.1. You can open this link in a web browser from any device that's connected to the Overdrive's wifi network, including your mobile phone if it has a web browser.
Enter your admin password and click OK to access the Overdrive's setup screens.
Hopefully you wrote the password down somewhere when you first configured your Overdrive, or you can also find it in the original contract you received from Sprint (which they also sent to my email address).
Click the "WAN" tab in the admin console, and choose the "4G only" option from the WAN Mode dropdown.
I found that selecting "4G Preferred" doesn't connect at 4G very often, and tends to stay connected at 3G even when a 4G connection is available. For example, the Overdrive was able to find a 4G network right away when I changed to "4G only", even though it was running on 3G before I made the change.
I don't see a huge motivation for Sprint to ensure that the device switches over to the 4G network if there's one available, because those bites don't accrue as part of the 3G bandwidth limit - meaning they don't get Spring any closer to being able to charge you overage fees.
The drawback to setting the Overdrive to only connect at 4G is that you will find yourself with no connectivity if there's not a 4G signal available. This won't be an issue if you're in a coffee shop somewhere near a 4G tower and you're not moving around the city.