How to extract 4-digit long numeric value?

I want to extrcat 4 digit long numeric value from thousands of cells.

What it is: Alps, 3 to 999 worth,. 78999-../ zit,8877 99-../
What I want: 8877 (because it is 4 digit long number)

position of 4 digit numbers in the cells is not always same.
Some cells have no 4 digit number I want blank output for these cells.

please ask if you want to know something more abouth this problem, thanks.