It’s all about lap times. I’m afraid I don’t have any direct experience with 4 speeds to share but I do have a suggestion. I never road raced a 4 speed Triumph, but rode many many dozens of road races with a 5 speed. The short stock first gets off the line really well geared 4.6 or so (20/46 using stock 2:1 primary) and is still reasonably good at 4.3, so I wouldn’t want to change that. Also, I never had a problem with the gap between 4th and 5th, however I was geared, it always seemed like that was the tightest split in the gearbox and never caused a problem. But the real speed limiter in road racing a stock Triumph 5 speed is the big ratio gap between 2d and 3d. In the garden variety road race hairpin, it can make second too short, and 3d too tall, and require a shift where you don’t want one. Charlotte’s Web at Barber, turn one at Miller
, International Hairpin at Daytona, and to some extent turns 4 and 5 at Roebling are examples. Early on racing my bike, when it was at a lower state of tune, with less torque
, this was a big problem. Fully developed at the end, I had 11-1/2:1 pistons, and twin plugged head, and longer pipes, and it could make acceptable power down low below 5000 in 3d, and I could avoid an extra downshift/up shift, which improved lap times a lot. I looked for a taller 2d gear for the 5 speed but could never locate one. That same big gap between 2d and 3d seems to show up in my street 70 Bonneville 4 speed. That makes me think a taller (close ratio) second would be a really good thing if that gear split is a problem in your 4 speed.