Constitutional Isomers
There isn't a good overall formula that I know.
You can look at C/H ratio and easily tell a bit here.
There must be at least one of the following, but not two:

carbon ring
carbon double Bond


(1) 1-Butene    

(2) 2-Butene which has two stereoisomers
cis-2-Butene 
trans-2-Butene  

(3) Isobutylene  

(4) cyclobutane  

(5) methylcyclopropane