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.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.