If f(3)=5,f(n)=f(n-2)+1 then what is f(4)? I don't know how to get f(4) because the function only gives an odd x value?
Examine $f(n)=.5n+3.5$. Does $f(3)=5~?$
Look at this page. You must scroll down to see the final answer. $f(4)=0$
Note that from the given
$ \begin{align*}f(3) &=5 \\\text{so that if }f(n)&=f(n-2)+1\\f(3)=&5=f(1)+1\\f(1)&=4 \end{align*}$
Now look at this page.
See a different but perfectly good solution where $f(4)=1$