We shall prove a new non-vanishing theorem for the stable cohomotopy Seiberg-Witten invariant [4,5] of connected sums of 4-manifolds with positive first Betti number. The non-vanishing theorem enables us to find many new examples of 4-manifolds with non-trivial stable cohomotopy Seiberg-Witten invariants and it also gives a partial, but strong affirmative answer to a conjecture concerning non-vanishing of the invariant. Various new applications of the non-vanishing theorem are also given. For example, we shall introduce variants λ k of Perelman's λ invariants for real numbers k and compute the values for a large class of 4-manifolds including connected sums of certain Kähler surfaces. The non-vanishing theorem is also used to construct the first examples of 4-manifolds with non-zero simplicial volume and satisfying the strict Gromov-Hitchin-Thorpe inequality, but admitting infinitely many distinct smooth structures for which no compatible Einstein metric exists. Moreover, we are able to prove a new result on the existence of exotic smooth structures.where [X] is the fundamental class of X i and ·, · is the pairing between cohomology and homology.Notice that the image of c 1 (L Γ X ) under the natural map from H 2 (X, Z) to H 2 (X, Z 2 ) is equal to w 2 (X). This implies c 1 (L Γ X ) ∪ e i ∪ e j , [X] ≡ e i ∪ e j ∪ e i ∪ e j , [X] ≡ 0 mod 2.Hence the S ij (Γ X ) are integers.The first main theorem of this article can be stated as follows: