This work shows a complete phase diagram of hard squares of side length σ in slit confinement for H < 4.5, H being the wall to wall distance measured in σ units, including the maximal packing fraction limit. The phase diagram exhibits a transition between a single-row parallel 1-{boldsymbol{square }} and a zigzag 2-hat{diamond } structures for Hc(2) = (2sqrt{{bf{2}}} − 1) < H < 2, and also another one involving the 1-{boldsymbol{square }} and 2-{boldsymbol{square }} structures (two parallel rows) for 2 < H < Hc(3) (Hc(n) = n − 1 + sqrt{{bf{2}}{boldsymbol{n}}-{bf{1}}}/n is the critical wall-to-wall distance for a (n − 1)-{boldsymbol{square }} to n-{diamond } transition and where n-{diamond } represents a structure formed by tilted rectangles, each one clustering n stacked squares), and a triple point for Ht {boldsymbol{simeq }} 2.005. In this triple point there coexists the 1-{boldsymbol{square }}, 2-{boldsymbol{square }}, and 2-hat{diamond } structures. For regions Hc(3) < H < Hc(4) and Hc(4) < H < Hc(5), very similar pictures arise. There is a (n − 1)-{boldsymbol{square }} to a n-{diamond } strong transition for Hc(n) < H < n, followed by a softer (n − 1)-{boldsymbol{square }} to n-{boldsymbol{square }} transition for n < H < Hc(n + 1). Again, at H {boldsymbol{gtrsim }} n there appears a triple point, involving the (n − 1)-{boldsymbol{square }}, n-{boldsymbol{square }}, and n-{diamond } structures. The similarities found for n = 2, 3 and 4 lead us to propose a tentative phase diagram for Hc(n) < H < Hc(n + 1) (n ∈ {boldsymbol{{mathbb{N}}}}, n > 2), where structures (n − 1)-{boldsymbol{square }}, n-{boldsymbol{square }}, and n-{diamond } fill the phase diagram. Simulation and Onsager theory results are qualitatively consistent.