I asked that same question a few months back. You might try doing a search of my posts for the full responses.
In summary, the answers I received indicated that some of the Imperial Lugers were shipped with two digits and others, especially in the last years of WW-I, were shipped with no numbers. I have neither seen nor read of grips marked with the full four digits, but my area of familiarity is limited to Imperial Lugers.
BY THE WAY . . . on which kinds of Lugers have you found the four digit grips?