由4x+y-4=0
mx+y=0
联立解得交点（4/(4-m),-4m/(4-m)）代入l3：2x-3my-4=0得
8/(4-m)+12m²/(4-m)-4=0
整理得m²+m/3-2/3=0
解得m=-1或2/3

答案：m=-1或m=2/3