Memory Module - matching sizes

[TomHerman]

Received a  Zbook15-G6, intentionally ordered it with 2x4GB modules,  and will be adding 2x32GB Samsung modules ( DDR4 ,  PC4-21300 , 2666 MHz, SO-DIMM 260-pin, non-ECC).  

The original 2x4GB modules in slots 1 & 2 are not easily accessible (under the keyboard).   Slots 3 & 4 are easily accessible when the back cover is removed.

This PC will be used for memory intensive, engineering applications.

Is there any known performance degradation, if capacity in slots 1/2  (2x4GB) does not match slots 3/4 (2x32GB) ?       

The add-on Samsung modules do meet the Zbook15's published specifications.

Web search info is inconsistent, and seems dependent on specific motherboards and chipsets.  The Zbook User Manuals and Service Manuals provide zero guidance.

Any references to authoritative sources would be especially appreciated.

TIA

[soccer_dan · Accepted solution]

assuming you are adding memory modules in pairs, you should be fine adding to slots 3/4

Quickspecs: https://www8.hp.com/h20195/v2/GetDocument.aspx?docname=c06305136

[TomHerman]

Thank you.

The HP spec states:

"Maximized dual-channel performance requires SODIMMs of the same size and speed in both memory channels."

What slot # pairs considered "both" channels?

1 & 2 ? (the pair under keyboard).     3 & 4 ?

Is it correct that 1 & 2 must match , and 3 & 4 must match, but  1-2 and 3-4 can have different sizes?