With Maxwell card (that include both the 750Ti, and 950), your 4,1 won't shows anything (black screen) unless you have the web driver installed and selected. That means you MUST install AND select the driver BEFORE you install the card, otherwise, black screen only (or you can use screen sharing to install and activate the driver from another computer). And almost any OSX update will cause "black screen" again, until you install the latest drive for that particular OSX version. A little more work to maintain to system functioning.
For Maxwell card, OSX 10.10.5 or later is required.
Also, for non Mac EFI card (that means >99% GPU you can buy from the market), it won't display anything until boot to desktop. Therefore, no boot screen available, and also means, unable to use the recovery partition.
For FCPX, AMD card usually work better (assuming the same cost), 7950, R8 280, 7970, R9 280X are the good choices. But all these card run quite hot (especailly compare to the Maxwell card), and draw more power relatively.
All these AMD cards are plug and play (work out of the box) as long as you have 10.7.5 or later. Easier to manage.
You can flash the AMD card by yourself to make it as a "Mac Edition card" (have boot screen), however, since you want to drive a 4K screen at 60Hz, you better leave it run without EFI. Which give's you exactly the same performance, but just no boot screen. Otherwise, you cannot boot with the 4K screen connected (Mac EFI for these AMD card not working well on 4K 60Hz mode, they work fine after boot, but can't boot at 4K 60Hz).