run a query in SQL

I have a table called "customers" in a database... i have a field in this table, called "zip_code"

zip_code varchar(255) utf8mb4_unicode_ci

zip code should be always 5 character... like "08001" or "28001" or "42001"

But i have certain cases were the field have been filled with only 4 character.. as "1001", "8901" or "1234"

I dont now how to write the sql sentence for this:

For all the data points in the field "zip_code" of the table "customers" with only 4 characters... add a "0" in the first place...

ie...
if the data is 8001... it should be 08001...
if the data is 1234 it should be 01234

taking care that there might be 28001 and 41234 that sould not be changed because they already have 5 characaters...

so i need the text to Run SQL query on table petplandb.customers: