Like Scott said, multiple batteries connected together create a "bank". If you have isolated batteries, such as a starting and stereo bank, and you want to charge them both without using a battery switch to connect them together, then you need multiple bank charger.
As for the output of the battery charger it will all depend on how quickly you want to recharge. If you have the time, slower is better.
The four optimas will probably combine into around 340 amp-hours. IF they were completely dead, and you wanted to charge them back up in a single night (so you can go back out on Sunday morning), then you would need to get 340 am-hours in 12 hours, which would require a ~28 amp charger.
If you have all week to charge the batteries back up you will be much better off to get a charger that is 10 to 20 amps instead.
More important than the amps of the charger is that it be a "multi-stage" inteligent charger. The old fashion dumb chargers taper their charge off significantly so even if they are rated at 30 amps they won't deliver it into anything other than a compleltey dead battery. This will dramatically increase charge time.
The inteligent multi-stage chargers charge faster and treat the batteries much better.