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"#.
