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 C4H10; 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, C3H5OH versus C3H6O2 in propanoic acid.
