Alkene 4:

Problem

Solution

Acetaldehyde 1 and 2-butanone (methyl ethyl ketone) 2 are the products of the ozonolysis of (E)-3-methyl-2-pentene. The formation of two different, mono-functional products indicates that the starting material was acyclic and trisubstituted. What we don't know is whether the alkene was of the (E)- or (Z)-geometry. Note that (Z)-3-methyl-2-pentene 3 also affords 1 and 2.