26．已知数列$\{a_{n}\}$满足$a_{1}=4$，$a_{n+1}=\left\{\begin{array}{l}{\frac{1}{2}{a}_{n}+n,n=2k-1}\\ {{a}_{n}-2n,n=2k}\end{array}\right.(k\in {N}^{*})$．
（1）求$a_{2}$，$a_{3}$，$a_{4}$的值；
（2）记$b_{n}=a_{2n}-2$，证明：数列$\{b_{n}\}$为等比数列．