Melting point and boiling point in isomers
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.