Need help picking a RAM upgrade for an old laptop, as the one I bought did not work

[Conjo]

Hi all!

So I have in my hands an old laptop, a Toshiba Satellite c645-sp4137l. It came with 2 GBs of ram, and it has 2 slots that support max 4GB each for up to 8 GB total (based on info I found).

The RAM that came with it is a Samsung m471b5773chs-ch9  (2GB, 1333MHz DDR3)

 

The one I got was a Kingston kvr16ls11/4 (4GB 1666 DDR3), and I'm not fully sure why it doesn't work properly (it appears in the BIOS and all, it just generates a lot of random errors when trying to be used by the OS)

I think I saw somewhere that the density of the DRAM chips can affect compatibility with olders laptops, and that's probably the only meaningful difference I can see between these two RAMs (Kingston uses 8x512M chips, Samsung uses 8x256M from what I found). The kingston should support the 1.5V because well, it's standard and it says so on this PDF on their site. And the ram should be able to run at a lower speed. I tried running them both together, or only the new one on any of the 2 ram slots on the laptop, it didn't work properly. It does work on another laptop though, so I know it's not the Ram stick that is faulty.  

btw, the RAMs I can buy can be seen here: https://www.solotodo.cl/rams?total_capacity_start=197532&types=130770&formats=130761&voltage_end=197737&ordering=offer_price_usd&, it's a site that compiles and compares prices from the stores in my counttry, kind of like pcpartpicker does