I played an euphonium with the four valves in line for twenty years and I didn't have any particular problems. Now, since 2007, I play on 3+1 euph: it took me a few months to synchronize the two hands, but then I must say that the latter is certainly better ( ring finger and little finger are activated together). I happened to play again on 4 in line rush and old habit was automatically returned. If you do the reverse passage ( from 3+1 to 4 ) you can safely operate the 4th valve with the index finger of your left hand.