some letters do look somewhat similar, such as the G's and the P and the R... but they are not the same, - things like angles, lengths, loop shape etc are what makes them by the same hand, not the overall shape - as they are very common.
also would they have only signed their first names?
id have expected a full name at least
although, if forged, surely theyd have added a john too... so that seems odd - like it coul dbe that the person only managed to get the other 3...john left early or something...?
you may as well get it checked out - lets face it no-one here really knows so dont take our word for it...