[goinbroke2]

Nothing to do with engine hp.
It's resistance.
In lower gears there is gear multiplecation and less resistance on the input shaft of the tranny.

Same reason an old 6cyl pick up will go up a hill in 3rd without slippage but in 4th it breaks free. The engine isn't making anymore power, it's just reaching the coefficient of friction. Steep enough hill and it will slip in third too.

It's been well documented that slower rpm/second will increase hp on a dyno because of less torque loss to rotational forces. In other words, it's not eating so much torque to actually spin up the rotating assembly. (big blocks more effected than small blocks)
And while I agree with you, I don't think this is the case here for slippage. Not saying it can't be, I just don't think the clutch is adjusted to just the fine edge, where a couple lb'ft gained by the slower rpm increase would effect it. The only increase in torque would be from the difference of rotational speeds of 3rd and 4th.

I would say the clutch needs to be tuned to stop the slippage. (actually it does regardless, but it's because the resistance applied in 4th is more than the clutch can accomodate)

Where are the clutch guru's? They will be able to explain must more concise than I.