I have both, race the gen 4 and track day the gen 5 but will likely start racing the gen 5 next round at the end of July.
For the record, 2011 I've had since the dealer hold was lifted back in 2011. 2016, I've had since February 2016; so both bikes pretty much as soon as they were available.
Money spent on the gen 4 thus far: Lost count at $13,500
Money spent on the gen 5 thus far: ~$3000
The bikes characteristics are drastic enough that I warrant having both. Hence why every time I try and sell my 4th gen, I always pull back the ad after taking it out for a session or on the street if I have the street plastics on it. Then I ride the 5th gen and am totally impressed all over again and kind of go wow, this is how far we've come.
The gen 5 is better out the gate then a gen 4; no ifs and or buts. The Gen 4 takes quite a bit in the suspension, braking and power department to put it on par with the gen 5 out the crate. The gen 4 weighs slightly less, but after you pull the ABS, race battery, pull emissions crap, KLEEN system, etc they are essentially the same. Even in the power department, unless you've done headwork on the gen 4, you'll still be down about 10HP compared to the gen 5. Unless you've done custom con-rods, pistons and knife edged the crank on the gen 4 it will rev slower than the gen 5.
Electronics on the gen 5 blow the 4th gen away. The electronics on the 4th gen were good for their time what they were up against. But now that nearly everyone has gone to a dedicated IMU, there is no comparison for actual bike situational awareness being calculated live and in real time as opposed to reactionary, simple software algorithms based on mechanical feedback from the motor, trans, wheels and rider input. The tuning options we have with ETVs compared to cable based throttle is huge especially with logging systems where we can log the actual ETV position in relation to the throttle give the gen 5 a nod over the gen 4. Quickshifter is a moot point for most. But the ability to have the auto blip downshift on the 5th gen is a relatively huge advantage as that 5-10% of mental capability you afford to making your approach to a corner to blipping throttle and rev matching you can essentially reallocate elsewhere, perhaps to your braking points. It's one less thing to worry about. More TC modes is an advantage as well. Launch control, like the QS again is moot as most people will put the woolich racing launch control in on the gen 4 if they need it.
I love both my 11 and 16 model but in terms of all around performance (brakes, suspension, motor) the 16 wins. But again, where I have both bikes currently set, I do notice a large enough gap between the two to warrant both. Maybe that will change more once I dig deeper into the 5th gen and that gap will grow and it will really start to outshine the gen 4? Don't know. But when it comes down to it, the best analogy I can give describing the differences between the two bikes is this... you're trying to remove a Phillips head screw from a pair of glasses and you have two different screwdrivers available to you; a precision jewelers screw driver and a typical screw driver that is slightly over-sized but bites just enough on the head. Yeah, the slightly over-sized screw driver will get the screw out, but the precision/jewelers screw driver will do it faster, more efficiently with less energy spent by you and less fatigue absorbed by your wrist all along with less damage to the screw itself.