Something to consider with the water bombers is the ability to get into and out of ravines and hillsides.
The Flip side is getting into lakes, reservoirs and harbors (calm coastal waters?).
Maybe the 4 engine planes have the same "field" performance as the twins?
In another thread one of our members had pictures of 3 Canadair's operating as a 'train', landing, scooping and climbing out in quick succession. Not sure if there were photos/videos on the other end. Maybe three drops just a minute or two apart are more effective than either one large drop and then a 10-20 minute delay to the next?
A lot may depend on each fire and/or terrain/fire load.
For some of these planes you need 2-3 crews for each plane. You need at least 2 crews for a long day to fly in daylight. If they try operating at night you need a 3rd crew. Some of the these planes have operated for around 48 hours in the early years (France or Spain?)