This may be just me, but I've run into a considerable number of bugs on Windows 10 already. If you don't mind it when shit blows up for no reason, or you're really into shiny new stuff, then it wouldn't be a bad idea. But that's probably not the case if you've come to post here about it.
If you ask me, if you're using Windows 8, I'd probably recommend upgrading. If you're on Windows 7, then you should probably just stick with it for a while (unless what I mentioned above applies to you). As far my experience goes, I'd describe Windows 10 as an upgrade in the sense that many new features or quirks were added, however the OS isn't distinctively "better" compared to Windows 7 in the way that Windows 7 was to Windows XP.
Alternatively, if you like having extra stuff like apps and Cortana and that stuff, then Windows 10 is good. If you prefer not to have any extra crap, and you want your OS to run as smoothly and as streamlined as possible, don't upgrade (from 7), ever.
Oh, and I don't think you'll have problems with 4 GB on Windows 10, although depending on what things you disable or leave on, your mileage may vary.
