Welcome to the forum and to the world of the euphonium!
As for the 4th valve mystery, check out the article linked below. It is intended to explain the compensating system, but also shows why you need a 4th valve.
http://www.dwerden.com/eu-articles-comp.cfm
Old Bessons and B&H euphoniums and legendary and are nice horns. They have a very solid sound and are fairly easy to play. The ACB will respond a bit easier and more consistently, especially in the very low register (which may not be part of your playing for quite a while), and will have quieter valves. Bessons are all sharp on the 6th partial (if you read treble clef, they are the F on the top line, the F# and G), so you might need alternate fingerings to fix the pitch. The ACB is better there, but not quite as good an octave lower. You can see that graphically on the link below. When you get there, you can select both the ACB and the Besson Sovereign 967 (which has a similar pitch profile to the horn you are considering).
http://www.dwerden.com/Intonation/