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We know, that cis isomers are less stable than trans isomers. What I don't get is why the boiling point is changing as opposed to the melting point. The change is melting point makes sense to me considering the stability.
Also the difference of mp and bp in iso and n-butene is not clear to me. How can I possibly explain the change in these physical properties? 
Melting and boiling point in relation to cis and trans isomers
The factor that affects melting point is, for the lack of a better term- packability. Molecules that are symmetrical can fit into each other more snugly and are difficult to melt. Therefore trans isomers, being symmetrical, always have a higher melting melting point.
As for boiling, molecules that are held together by any sort of intermolecular force are harder to boil. Cis isomers being unsymmetrical will almost always have a net dipole moment, as opposed to trans isomers where the symmetric nature cancels out the dipole moment. Therefore they are slightly polar and attract each other. Therefore cis isomers have a higher boiling point
Edit: I don't know why isobutene has a higher melting point.
