In this paper, we show that the usual limitations on the coefficient efad6538933bc5d053377d24dded" title="Click to view the MathML source">c=c(x) in the linear problem u(4)+c(x)u=h(x) with Navier boundary conditions and nonnegative right hand side h are not necessary to get the existence of positive or negative solutions whenever c(x) is a nonconstant function.