This work presents some existence, non-existence and location results for the problem composed by the fourth-order fully nonlinear equation u ( 4 ) ( x ) + f ( x , u ( x ) , u ′ ( x ) , u ″ ( x ) , u ‴ ( x ) ) = s p ( x ) for x ∈ [ 0 , 1 ] , where f : [ 0 , 1 ] × R 4 → R and p : [ 0 , 1 ] → R + are continuous functions and s is a real parameter, with the Lidstone boundary conditions u ( 0 ) = u ( 1 ) = u ″ ( 0 ) = u ″ ( 1 ) = 0 . This problem models several phenomena, such as, the bending of an elastic beam simply supported at the endpoints. The arguments used apply a lower and upper solutions technique, a priori estimations and topological degree theory. In this paper we replace the usual bilateral Nagumo condition by some one-sided conditions, which enables us to consider unbounded nonlinearities.