Extract first 4 digits from postcode from text column

deefunk1981 , One way is to search position of BT and then take 4 character

mid([column], search("BT",[Column],,len([Column])), 4)

deefunk1981 ,

Steps to follow:
1- Create a new column (by right clicking on a table and selecting new column option) and use the calculation given by amitchandak 

new_Column = mid([column1], search(" BT",[Column1],,len([Column1])), 4)

this will give you a new column with the first 4 digits of your postcode.

search(" BT"...) is going to search in your text if " BT" is there. If it is there then it will take first 4 characters(for which we have used ",4" in the formula) from BT to the right including BT.