If you only want "nd" or the integers between 1 and 4, inclusive:
Data>Validation
Allow: List
Source: nd,1,2,3,4

Alternatively, If you only want "nd" or any numeric value less that 5, try this:
Data>Validation
Allow: Custom
Formula: =or(A1="nd",A1<5)

Does that help?

Regards,
Ron