Need more details. Are you current Pro4 connected to your wifi or to a Hub/base. If a Hub/base what model? Only the Essential series can connect directly to your wifi like the Pro4. You can look at Arlo product page for details on the different cams.
I am thinking about purchasing the Pro 4 camera system but before I purchase it I wanted to know if I could add more cameras to the system?
You can add more Pro4 cams. If you get a Smart hub/Base you can add most cameras that connect only to the hub/base . If no hub/base and just connecting to your wifi only the Pro4, Essential series or Pro3 Flood light can connect directly to wifi.
You should research the details for each cam on the Arlo product page and FAQ here.