如图，$\odot O$是$\mathit{\Delta ABC}$的外接圆，$\angle \mathit{BAC}=45°$，$\mathit{AD}\perp \mathit{BC}$于点$D$，延长$\mathit{AD}$交$\odot O$于点$E$，若$\mathit{BD}=4$，$\mathit{CD}=1$，则$\mathit{DE}$的长是　 　．