Conditional Expectation of random walk square $E^{F_n}X_{n+1}^2$

Question

$X_n$ is simple random walk

It seems like $E^{F_n}[X_{n+1}^2]=X_n^2+1$

And $E^{F_n}[X_{n+1}^4]=X_n^4+6X_n^2+1$

I think the first expression is due to $X_n^2$ being positve

But how does it reach the second conclusion?