Can you screenshot your column settings in SharePoint and the error you are getting in the form please?

I was able to figure out the problem.  I used the Nintex form and a formula for the number of digits to enter. This was an easy fix. 

Hi Steve White‌,

Could you please post what formula you used? Will it work if the field may contain leading zeros?

Thank you!

I found a way to get the Nintex form to do what I need. I created a validation rule on the field (field name is PN4Digits) with the condition:

not(equals(length(replace(PN4Digits,0,x),4))

Cheryl glad you figured out a way to accomplish this. Here is what I did.

Thank you for your reply Steve White! My way is actually not turning out well. Is there a way to do this your way, but where the number of digits required depends on the value selected on another named control on the form? Our complete requirement is:

If ProductNumberType = A,  ProductNumberField1 must be exactly 4 digits, else exactly 5 digits. 

If ProductNumberType = A,  ProductNumberField2 must be exactly 4 digits, else exactly 3 digits. 

The main difficulty I'm having is getting Nintex Forms to recognize leading zeros as part of the string. Both ProductNumberFieldX fields are single line of text in the list settings.