I think the 4 duel limit is effectively your motivation... Maybe if the motivation was adjusted according to the dueling level difference between you and your target? If you constantly pick on 'weaker' players, your motivation is adversely affected - target 1 level below, you loose 110% of dueling motivation (what is it - 10 motivation/duel? so you'd loose 11), 2 levels below, 120%. If your target is a higher level, then you loose less motivation - 90% for a target who is 1 level higher etc. You could use a scale based on triangular numbers instead of linear - 1, 3, 6 etc.
Either way, that would help resolve the complaints about weaker players being seen as easy pickings and encourage dueling above your level. After all, isn't a Western trope the young gun taking on the top dog in an attempt to make a name for himself?