I used to run an isolator...but with that many batteries...your alternator will be working overtime, and probably won't charge any single battery very well.
The easiest solution is just to run another Perko switch (then wire the two switchs together at the common post). 4 batteries- 4 positions. That way, you can pick and choose which batteries are (not) being used, and which batteries are (not) being charged. I wouldn't charge more than two at a time.
However...you need to stay sober enough to switch the right batteries on- some guys don't like that responsibility.