3 valve v.s 4 valve
Jerry,
All your three valve fingerings work on a four valve horn. but a four valve horn gives you alternate fingerings and a bit more range.
The fourth valve drops the key of the open partials from Bb to F, a perfect fourth. Concert E2 is 1-2-3 on a three valve, and can be played as 2-4 on a four valve. F2 is 1-3 on a three valve horn, but can be played using just the 4th valve on a four valve horn. Likewise, C3 is 1-3 on a three valve horn, and can be played with the 4th valve. The fourth valve can also be used to control the sometimes-sharp open F3 and F4 concert (3rd and 6th partials).
Check Dr. Werden's discussion of the four valve compensating system at:
http://www.dwerden.com/eu-articles-comp.cfm
Bruce H Ward
Houston TX
Besson BE2056-2 4-Valve Compensating Baritone
1946 Conn 30-I Wonderphone 5-Valve Double-bell