Size Model and hp part number of suitable RAM

[Lalganj]

Dear Sir, I want to upgrade my existing RAMs . Details of System are :

RAM = hp Part No.L02855-971   ( 4GB, DDR4, 2666MHz, 288-pin DIMM  ) . 4GB RAM in each slot.

Slots Available in motherboard = 02 Nos.   

Motherboard Model =Base Board Product 86E9

System Model HP 280 Pro G5 MT Business PC
System Type x64-based PC
System SKU 7TE09AV
Processor Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz, 3000 Mhz, 8 Core(s), 8 Logical Processor(s)
Base Board Version A (SMVB)

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Please suggest Size and specs of RAM sticks I can replace with existing ones.

[SDH]

I'd fill all sockets with 8GB sticks unless you truly need 16GB sticks.  Here are the "After Market Options" part numbers you need. Note that Crucial (Micron's online outlet) does a good job of providing their proper memory for HP computers... just look things up. Most HP memory these days is coming from Samsung, SKhynix, and Micron. 

You have to get the right stuff.

[DGroves]

Crucial has exited the consumer memory market

their website no longer has any consumer memory kits listed 

they still offer crucial branded consumer SSD's however

for ram (and ssd's) check Kingston's website

https://www.kingston.com/en

[DGroves]

the posters HP 280 Pro MT motherboard only has two dimm slots

so "SDH's" recommendation of two 8GB dimms of the same specs/rank is the most cost effective

and byw,  your current 4GB HP memory part# L02855-971

is a single rank device, which can't be mixed with a dual or quad rank memory stick

Pins:

288

Rank:

1RX16

Size:

4GB

Type:

PC4-21300

Speed:

2666MHz

Voltage:

1.2V

Standard:

DDR4

the 1x4/2Rx4 refers to how the dimm is organised and it's known as the "Rank" and you can not mix ranks within a memory chanel so installing a 8 gb dimm that is 1Rx4 and a 8gb dimm that is 2Rx4 if placed together on the same memory chanel will not work

for example the z420 supports 4 memory channels

and each channel is two dimm's (total 8 dimm slots)

 so as long as you match the two dimms in a channel it will work fine

in other words dimm channel 1

place two 2Rx4 rank 8 gb dimms

and in dimm channel 2

place two 1Rx8 rank dimms

for a total of four dimms which will work fine