Stacking black cleavers is overrated and pretty stupid, you get more usefulness having other things, EG bloodthirster. While its useful, it stops being useful at 4. Even that, other items are probably going to give you CDR/Health/AD so its even more pointless. Plus, theres no sustain on top of that (lifesteal) that you could get from bloodthirster.
My kha'zix build that i'd rather get then stacking cleavers:
Boots of mobility
1 black cleaver, brutaliser but keeps upgraded last and not rushed to a cleaver.
Depending on how well i'm doing: bloodthirster or frozen heart
Bloodthirster / Frozen heart
Gaurdian angel / Maw
Its more effective then stacking cleavers IMO.