They are colourless gases that are present in crude oil as a minor constituent in quantities that are too small for viable extraction.
Butene is therefore obtained by catalytic cracking of long-chain hydrocarbons left during refining of crude oil.
[1] Butene can be used as the monomer for polybutene, but this polymer is more expensive than alternatives with shorter carbon chains such as polypropylene.
All four of these hydrocarbons have four carbon atoms and one double bond in their molecules, but have different chemical structures.
There are also cyclic alkenes with four carbon atoms overall such as cyclobutene and two isomers of methylcyclopropene, but they do not have the formula C4H8 and are not discussed here.