I ddn't try 4g but at the time i was there, 3g was fast enough (+100 kB/sec). The overall speed has increased in recent years, but unfortunately the stability of the connection has not. On some days, the connection will drop every few seconds, which especially sucks if you are using protocols depending on stable connection, like SSH.
If 3g is not working well enough, you can always use hotels as backup, especially the upscale hotels offer fast enough connection (but again, the stability may not be great).
In short, the connection in Ethiopia is ok for using Facebook, but for real work (like remote software development), you'll struggle.
