Molecules that have the same molecular formula, but different CONNECTIVITY are said to be "structural isomers". Structural isomers have distinct physical and chemical properties.
And thus "molecule 2" and "molecule 4" have a chemical formula of C_4H_10; however, clearly, they are structurally different in that their carbon-carbon skeletons are manifestly different. They should thus express different physical and chemical properties. You should look up their boiling points; straight chain "n-butane (4)" will be more involatile than "isobutane (3)".
On the other hand, for "molecule 1" and "molecule 3", we have different chemical formula, C_3H_5OH versus C_3H_6O_2 in "propanoic acid".
