Pull letters and validating cells below to see if the letters are same

I have been trying to make a field that selects the first 4 letters of a cell and then will see if the cell below has the same first 4 letters and so on until it doesn't.

For example

lets say
A1 = AHJJ
A2 = AHJJ
A3 = AHJJ
A4 =AHKK

I would like it to select the first three and not the fourth because A4 doesnt validate

each one of these are followed by numbers for example

A1 = AHJJ10
A2 = AHJJ11
A3 = AHJJ12

I would like it to select the top cell each time it finds a new letter sequence and display it in that cell as I need the one ith the lowest value which will also be the top reference.

I hope that is clear.

Thank you

Rick