Using $T$ flipflops design following $4-$Bir register $($ABCD$)$ which should work as follows: Each outpuls $($Ereept $1)$ Should be complemented if the value on its left is $1$ otherwise it should save its value. for example, if $B$ is $1$ the value of $C$ should be complemented when the clock comes for the left most output $A_1$ it should be set to $1$ at each clock pulse.