newbie - peoplecode to limit data entered into numeric field.

I am trying to set the field up to only accept numbers with a length of 4. I have the following peoplecode on the record field Sequence_5.  But when I go to the page to input data it doesn't stop me from entering only 1 digit. Can someone help me?

If IsNumber(SCH_STR_AUD_ACT.SEQUENCE_5) Then

   If SCH_STR_AUD_ACT.SEQUENCE_5 >= 1 And

         SCH_STR_AUD_ACT.SEQUENCE_5 <= 9999 Then

   Else

      Error MsgGet(20008, 3, "The Sequence should be 4 digits long.");

   End-If;

Else

   Error MsgGet(20008, 4, "Sequence must be numeric");

End-If;