Xeon E-series processor engraving meanings?

[xeonguy]

We can see 1) Brand, 2) model and generation, 3) stepping and base clock

But what is 4?
And what does that (E4) thing mean?

And what is 5?

Reason I ask is I read somewhere on STH that its important to keep the same model and stepping in dual xeon setups.

Thus, I was wondering if 4) and 5) is important or can be ignored when considering purchasing for the purpose of a dual socket setup.

 

[BlueFox]

#4 is the batch number (FPO). #5 is the serial number (ATPO). They're not particularly important unless warranty matters to you. Matching stepping is what matters and it's quite uncommon for any given model to have multiple steppings these days.