What happens during combat damage assignment if my 4 [M] unit has both Ki Barrier and Lotus Trap played on it this turn and my opponent has a 4 [M] defender?

You can order the replacement effects on your unit. If Ki Barrier is first, your unit will take no damage and you will have 3 damage prevention remaining on your unit. If Lotus Trap is first, your unit will take 1 damage and will have no damage prevention remaining on your unit.

With the Vendetta rules update, replacement effects that apply to damage in combat are streamlined: any replacement effect that would apply to combat damage being dealt will apply instead to combat damage assignment. This was already the case for prevention effects like Ki Barrier or Counterstrike, to prevent several strange edge cases. Now, damage increasing replacement effects will also apply during damage assignment.