The charger in your UPS is designed to charge 2 batteries. If you attach 4, it will:
a) either overload the charger hence never fully charging the batteries
if not overloaded, it will take longer (roughly double) time to fully charge batteries
The proper way of using more batteries is to do the following:
1) Install separate chargers for additional batteries (1 charger for 1 large capacity battery. If you batteries are less than 50Ah then you can use one charger for two. You need to check with the salesman about how much capacity the charger can handle). Under normal condition (Electricity available), the additional batteries must remain disconnected from UPS
2) When lights go out, connect the additional batteries to the UPS to enjoy additional backup time. Be careful of +ve/-ve terminals. As you have two battery setup, additional two batteries will connect in parallel to your existing two batteries. Parallel meaning +ve to +ve and -ve to -ve terminals
You can use a switching mechanism (can be auto or manual) to connect/disconnect additional batteries to UPS. In case of auto switching, some kind of relay is needed to sense the presence of electricity and engage/disengage batteries to UPS.