I need help with my project! (water lvl controll)

Hey,
Im working on my project with water level controlling.
I use transistor to start my water pump, but when I connect it on pin 4(arduino uno) it dosen't start the pump. When in use other pin's pump starts but when my sensor detects specific level it dosen't stops just get less power, I dont know. How to stop my pump? What pin should I use?
Please help me,
Thank you in advance!

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