I've designed charge controllers for high altitude balloon payloads, and they can be fairly sophisticated, but they are not charging lead-acid batteries. Lead acid batteries are pretty robust and tolerant of charging and discharging - connecting multiple ones in parallel is unlikely to damage them; and as pointed out, 12V lead acid batteries are not really 12V but >13.4V (as I recall). So, what's going to happen? Well charged one(s) will source current to the discharged one(s). But unless there is damage to a battery (say it is dead and shorted), nothing dreadful is likely to happen - eventually you might end up with four similarly charged batteries..
Considering the current levels available from typical solar arrays, four car batteries in parallel aren't going to be harmed in any manner, and the batteries WILL regulate the applied voltage to ~13.4V. (Batteries make great voltage regulators.)
I'd likely use diodes, because diodes are cheap (get a pack of six at Radio Shack for a few bucks) and work very well (they do cost you about .6V forward drop for a silicon diode) and because I Fear No Diode. Series resistors aren't required to protect such batteries, I suppose they might protect the solar cells (which I doubt will be harmed being shorted or heavily loaded in atmosphere <vs vacuum of space> where cooling is not likely any problem).
Under good conditions, sunlight energy amounts to about a kilowatt per square meter. That's a lot, but typical solar cell efficiency is only about 12-20% for commercial ones (newest research cells are now approaching 50% efficiency). A typical 150W commercial solar panel is about a square meter in size, and will produce about 1 kilowatt-hour per day.
Anyway, unless you have a very large solar array (very large!), it's unlikely to generate any very dangerous voltages or currents. But a 12V car battery can turn a ring or watch into a red hot object that will damage your flesh severely - in seconds.
(If you really want to do harm with the sun, a magnifying glass works quite well.)
L.
"Everything goes wrong all at once." -Quantized Revision of Murphy's Law