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Acids are compounds that release protons in solution, and the acids in which the acidic proton is attached to an oxygen atom are called oxyacids.

Therefore, oxyacids contain an X-O-H bond where X is the central atom.

These compounds give protons in solution, and the strength of the acid can be defined using the acid dissociation constant. The acid dissociation constant depends on the concentration of species in the dissociated and acidic form in solution at equilibrium.

The acidity of an oxyacid can be compared using the electronegativity of the central metal and its oxidation state. When the central metal is more electronegative, it decreases the electron density of the oxygen atom. This makes the removal of proton easier, and the acid strength increases.

The oxidation state of the central metal can be determined from the number of oxygen atoms in the acids and is directly proportional to the acid strength. As the oxidation state increases, the electron affinity of the central metal atom increases, thus making the oxygen atom less electron-dense.

The central atom in all given oxyacids is a halogen, and therefore, the acidity can be compared from its electronegativity and oxidation state.

In the periodic table, the effective nuclear charge on the valence electrons decreases down a group, increasing the atomic size. This causes a decrease in electronegativity, making their oxyacids weaker. Iodine lies under bromine in the periodic table. Therefore, the order of acidity of oxyacids can be shown as

$HBr{O}_3>HI{O}_3$

Also, chlorine is above bromine in the periodic table. Therefore, the order of strength of acids can be shown as

$HCl{O}_4>HBr{O}_4$

The acidity also depends on the oxidation state of the central atom. Bromine is in a +5 oxidation state in $HBr{O}_3$ and +7 oxidation state in $HBr{O}_4$. Therefore, the acidity order can be shown as

$HBr{O}_4>HBr{O}_3$

From the above conclusions, the decreasing order of acidity of the given acids can be shown as

$HCl{O}_4>HBr{O}_4>HBr{O}_3>HI{O}_3$