I'm struggling with this homework question:

Draw all possible isomers you can for C4H8O2.

So far I have three esters and butanoic acid, is that correct? I'm not sure if I completely understand this question.

Butanoic acid is one.

The question asks for you to keep H at 1 bond, C at 4 bonds and O at 2 bonds and draw structures for C4H8O2.
Don't forget the ethers and alcohols and double bonds. For example,
CH2=CH-O-CH2CH2OH. I expect there are a BUNCH of isomers.