There are many applications for a 4-bank charger. For example, you may have a 36 volt trolling motor bank consisting of three 12 volt batteries in series. You could have one bank wired to each individual battery and still have total battery to battery isolation since the seriesed circuit isn't closed with the trolling motor off line. That leaves you a fourth bank available for the starting battery.
However, just as Brett mentioned above, with multiple batteries in parallel they become one large battery as viewed by the charger which serves to circumvent the multi-bank multi-phase charger isolation/operation. Thus, there is no benefit beyond a 2-bank charger in this application. You may need to go with a 3-bank charger but only because the capacity dictates that a charger that large is not available in a 2-bank model.
What you want is a 2-bank charger that is the right current capacity to match up with the total battery reserves. And, you want a charger that is distribute on demand so that it will deliver a large percentage of its overall current capacity to the bank most in need.
It would be helpful to know what chemistry all your batteries are plus the specific amp/hour capacity of the batteries. From there, Brett, myself or others can tell you the exact type and size of charger you need. This charger should be available from a number of good manufacturers.