Digit3 can only be 3, 6 or 9 (as it's 3 x Digit2).
Digit2 can only be 1, 2 or 3.
Digit1 can only be 2, 4 or 6.
But, since Digit4 is two thirds of Digit1, Digit1 must be 6. Making Digit4 4 and making Digit3 9. Making Digit2 3.