My pc on idle uses about 2gbs of ram... I really doubt with 4gbs you would be able to run the game. Do yourself a favor and spend the $30 for the extra 4gigs
Use Crucial to determine what you need for a good price.
http://www.crucial.com/
I like gskill its cheap and works good. As for what I would recommend just find the ram that fits your price range and go with the best timings!!! Make sure you buy using your computer specs nothing worse than buying DDR3 when your computer uses DDR4
You will definitely need more ram. Your operating system will use 1-2GB of it anyway, leaving just 2 or 3 for the game. It will cause stuttering and choppiness. Its easy to replace, just buy the same type (DDR 3 or 4). You could get from amazon on next day or any component retailer. It's plug and play, if it is advertised as 'XMP' or something similar, you may need to go into bios at startup (f9 or del usually) and click the RAM profile as advertised to get the most out of it (it will still work regardless, just maybe better)
In regards to speed, there is little difference between 1333mhz and 2400mhz, just get an average price one from reputable brand.
If you want suggestions let me know your current ram settings (can download CPUz, really lightweight software which will tell you your current timings and speed under memory section)