4.
设\(x\),\(y∈R\),向量\( \overrightarrow{a}=(x,1)\),\( \overrightarrow{b}=(1,y)\),\( \overrightarrow{c}=(2,-4)\)且\( \overrightarrow{a}⊥ \overrightarrow{c}\),\( \overrightarrow{b}/\!/ \overrightarrow{c}\),则\(| \overrightarrow{a}+ \overrightarrow{b}|=(\) \()\)