Testing for valid numeric in an alpha field

I know how to do this in RPG but not Cobol:

I've got a 15-character alpha that is supposed to contain a 4-digit number 
left-justified.  In my
Cobol program, VNCOMPANY is a 15-character alpha field and WS-VND-COMPANY is a 
4-digit numeric. 
When I do this:
    MOVE VNCOMPANY TO WS-VND-COMPANY.
and VNCOMPANY contains '40             ', I get a decimal data error.

In RPG, I'd use a MOVEL and a TESTN.  Off the top of my head, the only way I 
can see doing this in
Cobol is to check the first four bytes of the field individually to be in the 
range of '0' through
'9'.  (Bytes 5-15 would have to be blank for our validation purposes.)

Any suggestions?

TIA, Dan

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