Yes. It is possible. The common configuration is to have the slingbox configured with a wired network and to watch the stream on a cell phone. In common terms, we speak of the 'upload' rate for the slingbox in this scenario. The quality of the feed is dependent on this 'upload' rate. For most wired systems, this 'upload' rate will support reasonable quality. The 'download' rates to the device being used to view the stream are typically higher.
If one is 'uploading' over 4G, one has to be aware that the 'upload' rate is for optimum conditions. Under sub-optimal conditions, the transfer rate is lower and intermittent. My experience is that the cellular provider will literally drop the calls/connections without notice and equipment may need to be restarted to get reconnected to the network.
One also has to consider that 'unlimited' bandwidth rates may have port filtering. One cellular provider I have tested does in fact filter ports and so not all services are available. My experience is that one can not ask the cellular provider, one can only test to see if it works. Further, if it stops working, it could be that the network has blocked the port.