Sorry for the delay. I note from my little black book that I suffered from serious 4th gear 'lag' on the standard 125* gears and sprockets and have noted that I changed to a Vega 46 rear sprocket and J 14 tooth front, with a 75 link (half link) chain. I'm assuming that this was retaining the J125 box but can't confirm 100%. My note from the sprocket change is, "Feels pretty good in 4th with no lag.", so whatever it was seemed to work OK

I think that this simply reflects that the J's small carb, restrictive pipe and plodder porting could cope with the 5.75 4th ratio, whereas the introduction of 20mm carb and revvier pipe benefits from a lower 4th ratio to allow the motor to benefit from the additional revs... much in the same way that LI125 5.65 was lowered to 6.32 for the extended rev range and power of the LI125S/DL.
Feel free to work out the ratios to see what they might be with the various sprockets as it just might be that mine is a Vega box with the J front sprocket... I really can't remember and my note keeping doesn't seem to be up to much! I'll update this if I figure it out

Adam