Thank you for the time Trouble 
So the information follows:
a) Native Windows 8 and Windows 10 intall
b) Same GPU ( Nvidia GTX980 ) with latest Nvidia Drivers
c)  They are both connected by Display Port
Thank  you 
So my guess is that i need to make a choice... where choice 1 is get the 4k display at the right scalling and live with the different scalling on the 2560x1400 ; b) Use both at 100% and live with the super height 4k resolution at 100% scalling... but that's just me being negative ( or realistic 
 )
Again, thank you for your time.
Pedro S.