My mac keeps changing screenshot format to .tiff I use shift + control + command + 4 a lot. 
The default fileformat in my case is .tiff.
I have tried to change it to .png using these setting: 
https://www.laptopmag.com/articles/change-default-screenshot-file-format-macos

And I have tried to restart, but it does not seem to save the file in png in the clipboard which is what I'm interrested in.
I have a Macbook Pro 15¨ (with touchbar) from 2016. I'm running macOS 10.13.3.

Kind regards,
JJ
Try using Terminal, type the following command into Terminal and press Enter. This changes the file format to TIFF:

defaults write com.apple.screencapture type tiff;killall SystemUIServer