Best external sound card iMac 4K

http://www.uaudio.com/interfaces/apollo-twin.html

I absolutely LOVE my UAD Apollo Twin duo. Connected via Thunderbolt. Drivers 8.4.1 are great. Their plugins are awesome as well. With the current driver you should be good though it's not completely tested with EL Capitan. I'm guessing by NAMM they should have something ready. Though if you want to spend a little more money UA has a killer special going on.

http://www.uaudio.com/apollo-satellite-promo

Seriously two thumbs up!!!

It really depends on your requirements: do you need a ton of analog i/o? lightpipe? preamps?

With regards to brands, you can't go run with Apogee, UA or RME to name a few. Personally, I use an Apogee Quartet since I don't require many analog inputs. That said, the UA Apollo Twin is a nice bit of kit as well.

Cheers.

Nice, that will definitely save you a bit of coin. Assuming you have a properly treated room and a sweet set of monitors, the Twin Duo should do you right. For ITB mastering, UA is certainly not short on quality plugins.

Enjoy!

It's a great little box with a good rep—definitely great at that price point. I'm certainly no expert, but I would say you're wise to spend the most of your budget on room treatment/monitor fitting. The most expensive audio interface on the planet won't do you any good if your room isn't up to par, especially if your concentration will be mastering. That's just my two cents.