Constitutional isomers for four carbon thioesters
One way to break things down in by the number of carbon atoms on either "side" of the thioester.

There are 4 carbon atoms to work with, of which one will be taken up by the thioester moiety.  Thus there are three left to work with.  

On the carbonyl "side" of the thioester, between zero and three carbons are possible.  However, on the sulfur "side" of the thioester, at least one carbon atom is required; otherwise the compound is not a thioester but a thioacid.  Thus the possibilities for carbon atom distribution are:


one carbon on S side, two on CO side.
two carbons on S side, one on CO side.
three carbons on S side, zero on CO side.


You have drawn the only structures for the first two possibilities.

For the last possibility, there are two possible ways to attach a three-carbon alkyl chain to the thioester sulfur:


as a $n$-propyl group
as an isopropyl group


Thus, the four possible thioesters are:


methyl thiopropanoate
ethyl thioacetate
n-propyl thioformate
isopropyl thioformate