Perfect. So, your system could take up to 32GB of RAM.
I'd argue that's probably a little excessive for your use, no point spending money if you're not going to get the benefit.
RAM is ideally installed in matched pairs. You've a couple of options:
1) Buy 2x 2GB sticks to match what you currently have. That will take you up to 8GB of RAM. This is the cheapest option
2) Buy 2 4GB sticks to upgrade to 8GB, or buy 4 to upgrade to 16GB.
I wouldn't go beyond 16GB personally.
The above might sound a bit confusing but here's what I've found - RAM kits where you get 2x sticks included.
These
should be compatible with your machine, but occasionally you can get weird incompatibilities - sometimes it's trial and error somewhat unfortunately. However I have found a couple of options that should work fine with your machine as they match what you currently have.
Note I've never heard of "Duomeiqi" but usually the chips come from one of only a few factories that make it and for most use cases it should be fine.
https://www.amazon.co.uk/DUOMEIQI-1600MHz-PC3-12800-Unbuffered-Compatible/dp/B07B9SSM4X - 8GB
https://www.amazon.co.uk/DUOMEIQI-1600MHz-PC3-12800-Non-ECC-Unbuffered/dp/B08NJRXVW2 - 16GB
Installing the RAM is fairly easy, but you have to be a little more delicate than you would with the SSD so if you're not 100% comfortable working around electronics it may be worth getting a professional to do this - ask how much they would charge for RAM or try a couple of local retailers. I'm searching Amazon UK but if you're outside of the UK this might not be suitable.