Your MB is the lga1150 type MB, you can use any 1.5v 1333mhz or 1600mhz DDR3 RAM. If you use higher speed RAM, like 1866mhz or up, they will run at 1600mhz only, because 1) your MB can't oc. 2) The i5 4460 can support either 1333mhz or 1600mhz RAM.
http://ark.intel.com/products/80817/Intel-Core-i5-4460-Processor-6M-Cache-up-to-3_40-GHz
So
1) Install the "cpu-z", and you can find the memory timing. Usually OEM will use the 1333mhz one.
2) After getting the memory timing. then you have two choices.
Buy the other 4GB RAM that has the same timing/speed as the old 4GB RAM. And you may or may not have problem to use with other old RAM, because there is no guarantee they will work together, when you mix the RAM even they have the same timing/speed.
Buy the new kit either 1600mhz cl 9/cl 10 8GB or 16GB RAM, we will recommend to buy the either 8GB (2X 4GB) or 16GB ( 2X 8GB).