y=sin^4(x/4)+cos^4(x/4)
=[sin^2(x/4)+cos^2(x/4)]^2-2sin^2(x/4)cos^2(x/4)
=1-1/2sin^2(x/2)
=1-1/4(1-cosx)
=3/4+1/4cosx
y′=-1/4sinx