SLI Titans will do absolutely nothing for viewport performance. SLI anything in fact does nothing for practically any DCC app you can care to name with some very rare GPU rendering engines making exception.
The K4000 is a dog, a Titan will run circles around it in most applications, but you seem to have made your mind up on artificial benchmarks and reviews made on old drivers by websites employing journalists with not the faintest idea about DCC apps (TH is one of the greatest jokes ever inflicted on humankind).
Mixing GTX and Quadro cards almost always ends in tears, go ahead and do it at your own risk if you wish, but make sure you can return either card if you do and things go wrong.
It’s theoretically possible, and with some driver combinations it works, but more often than not it won’t and you frequently get stuck with a non-ideal driver for either.
Buy a Titan if you need double precision anywhere, or if you need the 6GB of ram, or a 780 if you don’t need DP and are happy with 3GB, see how it does, and if it can’t cope with the load then buy a k4k or a k5k (making sure you CAN return it on buyer’s regret for no loss) and see if that improves the situation. Done.
Chances are you will find out getting both to work will be annoying, if possible at all, and that a k4k will make little or no difference with Nitrous, or some times even perform well below a Titan, especially if abundant amounts of VRAM come into play.