As I recall (vaguely), the 67C4 is one of Yamaha's takes on the Schilke 67. I know I've tried it, but can't quite remember the details. The 67 always seemed a bit big for me, and the 120S always sucked the air out of my lungs. I just don't get along well with the big tuba mouthpieces -- particularly the Helleberg styles.
Unless for some reason you're committed to a Helleberg, you might try a German (bowl) style and see if you like it. Miraphone has a good bunch -- but pricey. Bach has the 25 (which is maybe a bit small for what you want), and of course the 24AW (yuck). Perantaucci also has a good bunch, and I've used a couple of those in the past although the descriptions of them can be misleading. When I was still playing BBb, I'd given up on the Hellebergs except for the Schilke 66 which remains my goto-never fails-sounds good general use piece. Now that I'm playing comp Eb horns and don't have a BBb, I'm using Wicks, although on my little Buescher I tend to use the Schilke 66 (or the Miraphone TU-17, or the Bach 25).
Several places will send you mouthpieces on trial, which will typically end up costing you the shipping costs. If you in the mouthpiece exploration madness, it's a good way to go. Take it from one who knows.
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)