\[\Phi^{(4)}\left(a,b;c,c^{\prime};q;x,y\right)=\sum_{m,n\geq 0}\frac{\left(a,b;%
q\right)_{m+n}x^{m}y^{n}}{\left(q,c;q\right)_{m}\left(q,c^{\prime};q\right)_{n%
}}.\]