The RAM upgrade would help. An SSD would be way better but of course the price is heftier. eBay has some good prices on SSDs though. You shouldn't be having too many problems currently on 4GB of RAM. Honestly, it sounds to me like it's time to format the hard drive and start with a fresh install of OS X. Which version are you running? You could back your pictures, etc. to a flash drive to save them before you do it. I refurbish Macs all the times and it's amazing the difference in performance you get from doing an erase and fresh install.
Well you need to do a fresh install. If you have Yosemite, you have to create a USB boot drive or boot from the recovery partition, use disk utility, erase and fresh install. This is of course after you've backed up all your stuff to a USB drive or external hard drive.
So here's the two methods:
1. Turn off the computer, hold down the option key when you hear the chime. Two hard drive icons will appear, the second one should be your recovery partition. You would click that, go into it, use disk utility to format your drive, then do a fresh install of Yosemite. The catch with this method is you have to be connected to the internet because it's going to download all of Yosemite and then install so it could take hours.
2. Create a Yosemite install USB. The simplest method is to get an 8gb or larger flash drive and download this app - http://diskmakerx.com. It's completely free and does everything you need automatically. The catch here is you need to have the Yosemite install app downloaded. If it's in your applications folder, then you're fine and can proceed to use the app. If not, you need to download it from the App store. Once disk maker has done it's thing, you will have Yosemite USB installer. Same thing, plug it in, hold down the option key and now it'll be the third icon over and most likely will be orange with a USB logo on it. Format the drive with disk utility and then do the install.
So with both methods your ultimately needing to download the OS from the server. The only advantage is that with number two, you have it for future use and if your internet kicks off with method one, you may have to start the install again.
I can help you further if you need it.
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