Hi OP,
You've got an awesome rig! Congratulations for it. Would it be worth overclocking i5 4690k to 4 GHz? My answer is YES. It is also to be noted that you can overclock this processor up to 4.5 (or higher) GHz, provided you have all-in-one liquid cooler. With a Vcore of 1.33 V and 48x multiplier, you can enhance its performance to its peak. It is pretty obvious that you would see the uplift of FPS in games by 10% (approximately). It has to be praised.
Secondly, the stock power usage would be increased by 40+ watts. So, make sure you are good to go with the power consumption after overclocking. If you own a good liquid cooling like h100i, this "Devils Canyon" can have its temperature rarely exceeding 60c while playing games and 75c while stress testing with BurnInTest. Overall, there will be a significant increase in the performance of the processor which will help you to play games with highest/maximum frames per second with no room for stuttering, choppy graphics, and other lags.
Liquid Cooler Recommendation: Corsair H100i.
Air Cooler Recommendation: Noctua NH-D14.
Both works well to an overclocked 4690k. For stress testing, I recommend PassMark's BurnInTest and prime95.
In addition to all of these, your games will play even better (performance-wise) if you are able to overclock your card.
Edit: I can see that you own Corsair 200r case.
For fixing h100i, you may need to drill 4 holes. Here's the link for it:
http://www.overclock.net/t/1144409/h80-h80i-h90-h100-h100i-h110-case-compatibility-thread-page-1-for-full-listings/940#post_20446490 It is like installing the cooler with mod. Make sure you are not getting the cables pulled so hard.
For fixing NH-D14, you do not need to drill anything. You will definitely have a lot of headroom for it.