When you “Play in new window” with a resolution that doesn’t fit your monitors resolution. The preview window squishes to accommodate for the smaller resolution. This may be a “fix” in 4.10. As in 4.9 it did not do this. But now taking screenshots of my iOS games in 4.10 is no longer possible without a 4k monitor.
This is really detrimental and if this was a “fix” I bet most developers would like it unfixed.
While you wait for an official answer about this, a temporary solution is to hook up the r.SetRes console command in your Player Controller:
Hi Bpop -
How are you setting your screen size/resolution? I have tested in 4.10 and setting the Play in New Editor Windows size in Engine Preferences to a smaller size as my monitor is not causing any distortion when I take screenshots or in the actual window.
Can you tell me what your exact setup, what size are you working with as well as what resolution your monitor is set to as well?
Thank You
Eric Ketchum
Hi Eric,
I’ll explain the exact problem. Apple requires 5 screenshots of resolutions up to 2732 x 2048 (For Portrait) when you are submitting to the App Store. My monitor can only go up to 1920x1080 resolution. In Unreal 4.9 I was still able to play in new window, and take screenshots of in 2732 x 2048. In 4.10 however, if I set it up properly in Editor Preferences>Play>Play in new window the size I need. When playing in a new window it squishes to fit my monitors resolution. Changing the resolution of the play in new window resolution and making it so I am unable to take screenshots with the resolution required.
I hope this helps in describing what is wrong, It’s not with the lower resolutions, but with the more extreme resolutions that Apple requires to have screenshots of.
Thank you,
Brandon
Thank’s for the answer Triblade, However when I applied this at a higher resolution, something I just explained in a reply to my original comment (Sorry for leaving out info) It still squishes the screen and I’m unable to take screenshots at high resolution that the App Store requires. 
Hi Bpop -
I am getting the same truncation you are describing in both 4.9.2 and 4.10, but the good news is there is a simple solution to get you the high resolution screenshots you want.
In the Editor Preferences for Play in New Viewport, set the resolution to 1366 x 1024. When you Play use the console command HighResShot 2. This will take a high resolution screenshot at your required size of 2732 x 2048.
Thank You
Eric Ketchum
I’ll try that as soon as I can 
Neat little hack but awesome!