i7 4790k + gtx 1070 bottleneck?

Looks fine to me? A 4790K + 1070 is a pretty strong, balanced pairing.

Looking at the video though, I can't tell; is that GPU utilization, or GPU fan speed sitting ~40%?

I believe it's utilization, so you have a CPU running 50-60% across all cores/threads and a GPU ~40%.
The only "bottlenecks" in that situation is your resolution &/or in-game settings
 
What settings are you playing on? I really didn't see anything abnormal there. You never drop below 100fps once you load in, and your gpu and cpu are staying at the right clock speeds. A bottleneck is usually occurring when either the cpu or the gpu are running at 100% while the other is not, but it really depends on the software you are running. I don't see anything here that suggests a bottleneck, especially considering that the i7-4790k and the 1070 pair pretty well.
 
Do you have Geforce experience? You should optimize fortnite using geforce and see what settings it uses. Everything looks normal in the video you posted, but I do agree that your fps seems kind of low for your settings being on low. What are your resolution and monitor specs?
 


So this normal? The strange thing for me is that before reinstalling windows I remember having better fps. I'm playing at low settings

 


1920x1080p, 144hz Philips monitor
 
There's only so much demand "low" settings can put on higher end hardware. Something does strike me as wrong, as far as your actual framerate though...... IIRC, a 1070 can do >200FPS on low settings 1080p/60Hz. CPU/GPU utilization remains (fairly) low though....

It might be worth running HWMonitor in the background. Then you can verify the info you see in your OSD from a secondary source. Temps, utilizations etc.....

You're running >100FPS all of the time...... Can you crank those settings up to High/Epic and see how your system responds? Does your utilization shoot up to (near) 100%? A 1070, IIRC can do >60-70FPS average on Epic. That would give us an idea if it's hardware related, or something specific with the game.
 




In epic settings usage goes to 85~99%..
WYDnED6

HW monitor print when running fortinite https://imgur.com/a/WYDnED6

 


And what is your fps in game on epic?
 

average of 120~140, but sometimes it just drops to 80~70