West Ham are on track to finish in the top half of the Premier League table this season and are looking to increase the quality of the pool of players they have at their disposal.
The co-owner says that boss Sam Allardyce will be handed a significant amount of money to spend on his summer transfer targets.
Sullivan, speaking in the matchday programme, said: "Our planning has already started to make sure next season is even better than this one.
"A crucial factor of that will be recruiting the right players in the right positions.
"Our aim for the coming season is to bring in four or five players that will strengthen the squad and provide Sam Allardyce with greater options and variety next season.
"We will be aiming to bring in at least one proven goalscorer to further bolster our attacking threat."