Puzzle ZIQX 

Workings

Hint
What can digit 4 be?

Answer
6849.

Reasoning
Digit 4 must be a multiple of 3:
   ???3
   ???6
   ???9

Digit 3 is one more than Digit 9 ÷ 3:
   ??23
   ??36
   ??49

Digit 2 is twice Digit 3:
   ?423
   ?636
   ?849

But Digit 4 is one more than Digit 2, which only leaves:
   ?849

And Digit 1 is three smaller than Digit 4:
   6849.