In Excel you can use $
signs before the column or row references to make those references "absolute" (rather than "relative"). For example if you use =A$1
then the 1 doesn't change when you copy down. If you use =$A1
then the A doesn't change when you copy across. If you use =$A$1
then neither changes whichever way you go.
So for your case you need to use
=T6-T$4
when you copy that down T$4
doesn't change