Delete last 3 with filter applied

Hello,

1. I would like to filter column "O".
2. Filter certain components
3. Apply the following formula: =left(cell,(len(cell)-4)) to delete the last 4 text only to the filtered components in column "O"

I can get it to work only when it's not filtered. Basically I need to delete the last 4 text only to some of the cells not all. Any help would be most appreciated!