Is it possible to go beyond Super Saiyan 4?

no but legendary super saiyan makes it's user grow stronger for example broly is a legendary super saiyan so if a legendary super saiyan does not die he will get stronger than super saiyan 4 gogeta

it depends who is stronger at the bigning of the fight. like the guy above me said if ss 4 gogeta fights legendry super saiyn broly he will be stronger at the beging but if he dosent kill him right away broly will eventuly match his power the surpase it