Depends what you are using it for. For most people 4GB is plenty of system memory though don't open up tonnes of tabs in a web browser.
*My best advice is:
1) clone to an SSD if you don't have one
2) Upgrade to Windows 10 64-bit at some point if you currently have W7/W8.1
I would not upgrade to W10 yet though if things are running fine. I'd wait a few more months. Then I would:
a) upgrade to W10, then
b) do a CLEAN INSTALL of W10 (wiping all data off drive at start of install), then
c) ensure all drivers, programs installed and copy back any saved data.
*If you plan to get an SSD and also upgrade to W10 at some point I suggest doing both at the same time. Just do the upgrade on existing hard drive, then remove the hard drive and put in the blank SSD. Finally, insert the W10 install disk and let it write to the blank SSD.
W10 Install disc through the Microsoft Media Creation Tool.
(If confused ask for more info)