Unless you're using chiral HPLC, you won't see two enantiomers as two separate peaks.
So 4 peaks, on a non-chiral HPLC column, which have all the same mass (this should be checked by MS-search), are 4 isomers.
This doesn't imply they are stereoisomers (they could be constitutional isomers), and doesn't answer your initial question.
The NMR of your original isolated spot is what you need first of all.
You could also run the LCMS in basic conditions.