So i thought my formula would look something like this
=IF(A2, "/", then LEFT(A2, 4) ELSE, A2)
That formula's not quite right... it's a bit of a formula / VB combination!
I can't download your attachment - the zip file seems to be corrupted - but I think a formula that would suit you is this:
Alternatively, if you don't want to keep the original data and just want to remove the unwanted characters in situ, then the Text To Columns feature would be a quick and easy option.
Select the range > Data > Text To Columns > Delimited > Other = / > Select the 2nd column in the data preview pane and choose Do Not Import > Finish.