Only 4GB usable sounds like the RAM is faulty. I had faulty RAM once, it didn't show the full amount as being usable, and I had occasional BSODs and wasn't sure why until I ran memtest86 and it showed errors almost immediately. Note that even though your ram is failing/faulty doesn't necessarily mean you'll get BSODs, that'll only happen if the RAM gets unrecoverable errors.
It might not be that, but I would suggest running memtest86 for a few hours anyway (leave it overnight or even better, leave it for a full 24 hours)
4 GB should be sufficient to run Windows 10, but my media PC which has 4 GB RAM gets sluggish if I don't reboot for a while, so it might be a bit too little for it to run well. As long as I reboot every once in a while it works fine for playing videos in Kodi (including HEVC) which is mostly all I use it for.
Before I upgraded that PC to Windows 10 (wiping everything that was installed), it would be very slow at opening programs, Firefox was laggy, even after rebooting programs took ages to load up, it was pretty terrible to use. So it might be a good idea to reinstall Windows from scratch and make sure you get the latest drivers from the website of the manufacturers of the individual components, not from the OEM.
More RAM would definitely help though as I notice that with 4 GB it gets cramped very quickly when running multiple programs, even 6 GB gets cramped if I run a game and try to use a web browser while the game is running.