FCPX

None... When you import the videos into FCPX, also create proxy media to edit from.

In the view menu set it to Proxy.

When your finished, and ready to export, switch it back to Optimized/Original and Better Quality. If you forget to to switch and export the video. The video that exported will be from the low quality Proxy files and you don't want that...

I have the same computer that you have and this works best when editing 4k video.

iMac 27" Mid-2011

with these specs......

3.4GHz Quad-Core Intel Core i7

16GB 1333MHz DDR3 SDRAM - 4x4GB

2TB Serial ATA Drive + 256GB Solid State Drive

AMD Radeon HD 6970M 2GB GDDR5

You probably don't need to create *both* proxy and optimized media. Optimized media, especially for 4K, will gobble up huge amounts of drive space.

Speaking of drives: what you definitely need is a fast external drive.