Using recently proven modern tools, the authors reprove and strengthen two theorems about 4-manifolds obtained by the first author [J. Differ. Geom. 33, 335-356 (1991; Zbl 0839.57015); 357-361 (1991; Zbl 0839.57016)]. The first theorem states that there exists a contractible 4-manifold with boundary and an involution on its boundary which does not extend onto the interior. The second theorem gives two knots in \(S^3\) such that attaching 2-handles with framing \(-1\) yields manifolds which are homeomorphic to each other, but not diffeomorphic, even interiors are not diffeomorphic to each other. Thirdly, it is shown that all iterated positive Whitehead doubles of a Legendrian knot with non-negative Thurston-Bennequin framing are not slice. The ingredients of the new proofs are: Y. Eliashberg’s topological description of compact Stein manifolds, i.e. complex manifolds which admit strictly plurisubharmonic Morse functions (PC-manifolds) [Int. J. Math. 1, No. 1, 29-46 (1990; Zbl 0699.58002)], a result of P. Lisca and G. Matić on embeddings of PC-manifolds into Kähler surfaces [Invent. Math. 129, No. 3, 509-525 (1997; Zbl 0882.57008)] and the adjunction inequality of P. B. Kronheimer and T. S. Mrowka [Math. Res. Lett. 1, No. 6, 797-808 (1994; Zbl 0851.57023)].