Seperate cells by adding null line according to numbers

Hello excel forum readers!

I am looking for a time saver macro,pretty easy to make i guess,as the theory is not difficult...but i am too newbie to make it

So i have an xls that has like 20k lines on Column A!And i have to seperate the numbers...
I count the first 4 digits and I have to do it by adding a cell between them.

Example :
27289802
27289902
27289915
27289915
(add a null line)
27290202
27290302
27290316
27290402
27290416
27290431
27290701
27290703
(add a null line)
34100105802
34100105815
(add a null line)

As a said everything happens in Column A.

I would really appreciate if someone can help me...it's a living hell trying to do it manually!