Excel 2010 :: Macro To Trace Empty Cell

I need to write an excel macro to trace the valid PO# in my daily report.

Below is the condition:

If the PO# is an empty cell, the result return at column B as Not Valid

If the PO# starts with any character (example: FISHNH123), the result return at column B as Not Valid

If PO# is not starts with 3000 or 490 or 450(example: 3000823041,3000823037), the result return at column B as Not Valid

PO#
Status

empty cell
Not Valid

PO# starts with any character (example: FISHNH123)
Not Valid

PO# is not start with 3000 or 490 or 450
Not Valid

I'm using excel 2010..