In cases where generic non model specific plates are used the 4 digit number is just the consecutive number of that plate which could have made it on possibly 10 different models.
A 7 digit number example - C5030567 - translates, 5 = 1995, 03 = 3rd month [March], 0567 = 567th guitar built in March 1995.
An 8 digit number example - C03114373 - translates, 03 = 2003, 11 = 11th month [November], 4373 = 4373rd guitar built in November 2003.
Where things go astray is when the Jem line went to AANJ in 93'.
This is where you start seeing consecutive numbered plates on the remaining regular joint models that no longer indicate year.
Prior to 11/2011 there are 5 digits followed by a letter.