Is it possible to exchange the F1, F2 key functions for the F3, F4 key functions ? I was wondering if I can switch the functionality of the F1 and F2 keys (brightness down,up respectively) and pass it to the F3,F4 keys; subsequently switching the functionality of the F3,F4 keys to the F1, F2 keys. 
I want to keep the functionality of all the keys but swap their locations. 
Is that possible? 
My operating system is High Sierra V10.13.6
Thank you! (and sorry if the answer to this question is obvious!)
If third-party software is acceptable, Karabiner-Elements allows you to do this quite easily. You can manually assign each key's function independently, either to emulate another key or to activate a certain action, such as controlling the brightness.

Here's what these settings would look like, if I understand your question correctly: