I believe conditionnal and circumstance bonus to damage are ruled differently from bonuses to d20 rolls. While bonuses and penalties to d20 roll are limited within the -4 to +4 range, most damage bonuses can go beyond like all weapon traits that give +1 damage per dice rolled such as Charge, Twin and Forceful weapons as well as class abilities that do the same such as the Paladin's Blade of Justice and the Cavalier's Challenge.
Also the Monk's Diamond Fist feat hints that damage bonuses, even of the same type, should stack, as it turns the monk fists into forceful (+1 circumstance bonus to damage/dice) weapons, and for those whose fists are already forceful give a +3 circumstance bonus to damage on the second strike and +5 on the third, which if they don't stack would mean the additional entry is useless since it has no effect.
Altough this is still an unclear subject, it's worth reading the entries for Diamond Fists on page 103 and Other Rolls on page 293.
It hints towards the cap only affecting checks / saves and other d20 rolls.
