Answer
$K_f = 2~J$
Work Step by Step
We can use conservation of energy to find $K_f$:
$K_i+U_i+W_{ext} = K_f+U_f+\Delta E_{th}$
$K_f = K_i+U_i+W_{ext} -U_f-\Delta E_{th}$
$K_f = (1~J)+(1~J)+(4~J) -(3~J)-(1~J)$
$K_f = 2~J$
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