While streaming over 4G, Plex applies the Limit Cellular Data limitation (Settings > Quality > Video Quality > Limit Cellular Data). Based on the screenshot above that’s limited to 720p videos @ 2 Mbps.
If you want to avoid transcoding at all cost you might also want to disable the Automatically Adjust Quality option (this way you might experience a stutter if the bandwidth gets really bad but Plex won’t constantly attempt to optimize the streaming).
4G bandwidth will highly depend on tons of environmental factors (coverage, distance to the next antenna, capacity in a particular cell, number of “competing” users, walls/furniture/metal/…). So while you might not have a permanent/stable 45 Mbps connection, it should easily be able to deal with more than the 2 Mbps from your video quality limitation.
