RAM: Crucial
CT4G4SFS8266

Description: 4GB 2666 MT/S 260-pin DDR4 SDRAM SO-DIMM (PC4-21300), Single Ranked x8, 1024M x 64, non-ECC, 1.2V
Standards: DDR4, SO-DIMM (PC4-21300)

This is the "other" kind of DDR4, the kind that goes into laptops. The distinguishing mark is the "SO" in "SO-DIMM". It also happens to be necessary for Thin-Mini-ITX motherboards. This is because it lays down almost flat, to save space, and takes about half the space. In all other respects it is the same as the ordinary DDR4's. Pricing is usually about the same but not always.

Many people stay away from SO-DIMM, thinking its slower. But this is just a stereotype. Though there are subtle speed differences it is still basically the same super-speed DDR4.

To get added speed the RAM controler likes to divide each memory request between two memory modules. That of course requires two modules. But most motherboards will work just fine with one module.

Aside from that there's what's called "Single Rank" and "Dual Rank" modules. Single Rank gets its job done in one pass through its chips, while Dual Rank requires two passes. Generally, you'll only want Dual Rank in special circumstances or on odd motherboards. Single Rank will give you better performance in nearly all scenarios, and is the one to buy when you don't know what to do.