If it's the patent for the 4-valve system you're looking for, your failure to find it MAY be because it doesn't exist. This is purely speculation on my part, and I haven't done any searching myself, but I would not be surprised if the patent for the 3-valve system also covers the case of a 4-valve (5-valve, or ...) system.
This really depends on how the "claims" are stated. But I would expect that they'd be stated in such a way that the basic feature patented is the "recirculation" of the air path through the valves in order to achieve the desired effect. If this is so, then there would be no specific need to file a separate patent for a 4-valve system.
If the patent does exist, you should likely be able to find it by looking for patents that reference the 3-valve version -- since that would be "prior art" for any 4-valve version. The prior art would need to be mentioned and a clear account made of how the new mechanism differs from it in a significant way.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)