I see that you are stating you can stream your Slingbox using your WiFi connection but not while using your 4G network. For this issue there maybe a multitude of reasons why. The most simplest of answers would be your 4G network is not strong enough to stream live TV. Another reason may be the device, if the device is slow due to lack of storage then you would essentially come across this issue you are having as well. Due to the fact that you are able to stream on other devices points to a specific device issue. Aside from uninstalling and re installing the slingplayer application I would suggest contacting your cellular provider to see if there is anything they can do on their end to improve you signal strength.
Sling Media Support Team