Besides a) attenuation due to hand holding and b) change of the antenna characteristics due to bridging there's a third problem which really exacerbates the first two: the antenna of the iPhone 4G is highly directional. In other words, it matters a LOT which way you point the phone. Sometimes even small changes around it can make a big difference in terms of whether you get data or not.

You can test this out (assuming you've got access to an iPhone 4G) by running a speed test application (there are plenty in the App Store) while holding / pointing the phone in different ways. I can trigger signal loss even without holding the phone. No bumper whatsoever is going to fix that problem and this is plain and simple bad antenna design. I lose a lot more data when streaming radio on the 4G than what the 3G did even though the bandwidth is (potentially) much higher.