I prefer amp4. 3 is good but 4 takes it that last step. Both are good and if you need a balanced line out 3 is great too. Without another amp though 4 is more full sounding. No harshness but effortless in the highs, midrange is beautiful and full without being too warm and the bass is great. The soundstage seems to be more coherent and one to me. Amp3 while great had a trace of being a bit thin, in comparison to amp4. Each instrument is easy to place in the soundstage better and clearer with amp 4. I heard the difference right when I started playing music, no burn in with amp4. You can’t go wrong with either. The adapter from ibasso, 4.4 to 2.5 works great if that’s the route you take. I use the Vega with Reference8 cable from Campfire Audio.
Have fun!