Flag up missing result (1 of 4), miss a cell and start again

[Supersadie]

In a sequence of numbers 1-4 in a column, I'm looking to flag up the missing number the first time three of four have appeared (there will be repeats).
I want the next cell after the missing pattern to be ignored for tracking purposes.
So - if I get 1,2,3 in A1-A3, B3 should return "4", B4 will be blank and then we start tracking results again from A5.
This will make more sense when you see the attached.
Many thanks to kind souls for your help.