Automata and Complexity, Non Regular and Context Free Languages

$4.(8$ points$)$ Give a CFG for the language $\{\begin{array}{ccc}{a}^i{b}^j|& x,yinN,& x+y=i\end{array}$ and $4x+3y=j.$

$5.(8$ points$)$ Prove that the regular languages are closed under set difference. If $A,B$ are regular, then so is w not in B$\})$:$\}.$