Deppends exclusively on your current motherboard capabilities.
You can't add a DDR4 RAM stick in a mobo that support only DDR3 or vice-versa.
About speed, DDR4 are normally faster than DDR3 by a fair amount. To be real, the difference between speeds within the same RAM type are neglegibe to humans eyes, but on benchmarks they'll show a fair difference, IMHO you should pick the one that's faster while still keeping inside your budget.
More about DDR3 vs DDR4:
http://www.tomshardware.com/answers/id-1837908/ddr3-ddr4.html
About quantity, the sweetspot right now is 16GB, while 4GB being the bare minimum and 8GB being a budget option. Most games recommended specs ask for 8GB, so i wouldn't go lower than that, and if you wanna proof yourself 16GB is way to go.
EDIT:
As OP have stated he has a DDR3 capable MOBO, i'd advise getting some brand new 2x8GB DDR3 sticks kit