$y=4-5\sin^2 2x\cos^2 2x\\\hspace{0,25cm}=4-\dfrac{5}{4}.4\sin^2 2x\cos^2 2x\\\hspace{0,25cm}=4-\dfrac{5}{4}\sin^2 4x\\\text{Ta có: }0\le \sin^2 4x\le 1\\\to -\dfrac{5}{4}\le-\dfrac{5}{4}\sin^2 4x\le 0\\\to -\dfrac{5}{4}+4\le -\dfrac{5}{4}\sin^2 4x+4\le 4\\\to \dfrac{11}{4}\le -\dfrac{5}{4}\sin^2 4x +4\le 4\\\to \text{GTLN là }4;\,\text{GTNN là }\dfrac{11}{4}$
