TL;DR: Buy another RMT3170EB68E9W.
First things first: let's read what Lenovo specifies as
compatible standard memory:
DDR3-1600/1333MHz SODIMM × 2 (max 16 GB)
Time to break it down! Let's start with the hard and fast requirements, then get to the more difficult stuff:
DDR stands for "double data rate", a type of synchronous dynamic random-access memory (SDRAM), with the three indicating that it's the third of four major, incompatible1, revisions of the specification. There's a lot of implications that come as a result of this, but you really only need to keep one in mind: buy DDR3, anything else will not work.
Sidenote: the "double data rate" is where the 800 MHz DRAM frequency becomes 1600 MHz, since 800x2=1600. This is not the same as memory channels.
SO-DIMM, or "small outline dual in-line memory module", simply refers to the smaller size of the memory compared to their "normal" desktop DIMM counterparts. DDR3 SO-DIMMS have 204 pins. You will need a SO-DIMM module to work in your laptop. Though DIMMs and SO-DIMMs are electrically compatible (there are even adapters from SO-DIMM to DIMM), trying to squeeze a full sized DIMM into your laptop lies somewhere between absurd and impossible.
DDR3 offers three2 different voltages that modules can operate at:
- DDR3 or PC3 is the Standard Voltage memory module that operates at 1.50V
- DDR3L or PC3L is the Low Voltage memory module that operates at 1.35V
- DDR3U or PC3U is the Ultra Low Voltage memory module that operates at 1.25V
Your motherboard specifies the standard voltage DDR3; I'd recommend buying normal DDR3. Most DDR3L/DDR3U can operate at the higher 1.50V (check the spec sheet first though), but you'll be paying a premium for a feature you won't be using.
Memory speed is like the water speed in a pipe: the faster, the more throughput, the better. There's a decent amount of nuance to this, but basically, get whatever's cheapest. RAM speed (for most workloads) doesn't make enough of an impact to justify the additional price that comes with faster memory. Faster RAM can usually run slower, so don't worry about that.
The idea here is pretty simple: channels act as an exact multiplier for bandwidth! Imagine having one pipe versus two pipes. This tends to make a larger performance impact than speed (especially since the change in bandwidth is so much higher.) I'd recommend running your memory in dual channel; that is, having two compatible sticks. (This last bit is important; we'll come back to it.)
Timings refer to a grouping of specifications that relate to the the latency within a module of memory. This gets fairly technical fairly fast, but the general principle is lower is better, but it basically doesn't matter.
Phew, that's a lot to digest!
Since you've already got a 4GB module, it makes sense IMO to get another 4GB module. No problem then: find the cheapest DDR3 SO-DIMM, and away! Right?
No. Remember memory channels? You've only got two slots in your computer, so you have to run in dual channel (plus extra bandwidth!) The problem arises in that memory in the same channel may not play nice without the same speed and timings, and often even same type. While there's no hard-and-fast rule on this, you may experience some stability issues if you mix-and-match.3
In any case, your stick is common enough that it's not too much more expensive to just grab the same stick. For $25, I think it's worth it.
A final word though: your laptop does support 16GB of RAM. If you decide to upgrade to that in the future, get a 2x8GB kit.
- There are a few very select exceptions to this rule. There is some limited interoperability between DDR3/DDR4 and Skylake/Kaby Lake, and some DDR2/DDR3 interoperability on select LGA 775 platforms, but these are unusual exceptions.
- There's also LPDDR3 (different from DDR3L!), which draws even less power. However, these chips must be directly soldered to the motherboard (like in the MacBook Air.)
- If I'm reading your question correctly, you're mixing and matching right now; however, I'm going to recommend you refrain from that anyways.