Taking the question at face value, ALL the answers given are wrong [ :-) ] and your question and a subsequent comment do not match.
You say you have 4 x 12V batteries to make a 48V supply.
Your diagram implies (but does not state) that the panel is 24V rated.
In a comment you say
I don't think I know the PV array's knew voltage. I know it produces up to 18V in full sunlight though. – Wurlitzer Jul 10 '12 at 23:00
An 18V O/C panel is 12V rated, not 24V as per diagram.
Cheap and nasty solution:
Use a relay (or relays) to swap the panel between batteries. Either two positions if 24V panel or 4 positions if 12V panel.
A simple sequencer will be required to control the relays.
Very fast swapping rate reduces relay lifetime, and controller will take a time to settle so "dead time" % is higher.
Very slow swapping rate increases risk of imbalance.
If you swapped every minute or even every 5 minutes the long term differences will be minimal.
A good "real" solution is to use a 48V panel - or 2 x 24V or 4 x 12V panels in series.
Also suitable would be to use a 12V to 48V converter. Some MPPT controllers are available which will both up-convert the input voltage and also optimise loading to maximise panel power output.