Simple answer: two in series make 12v, two in parallel doubles the current.
The more involved answer is that for the cost of 8x AA nimh cells you could probably get 1 small(5ah or so) lead acid battery, which are the usual choice for solar power storage. This would probably last longer, because they tend to do better under constant standby charge from what I hear.
Better still would be lifepo4, but those need a current and voltage limited charge that needs to be very accurate, and multi cell packs need to have a balancer, or the cells will almost certainly be destroyed.
What I would actually do is to get a switching regulator module from eBay for a few bucks, put all 4 panels in series, and use the eBay regulator to drop that down to 13.5v(the lead acid float voltage) and feed that into a lead acid battery through a shottky diode.
Or possibly even get a commercial solar lead acid charger panel if I found one for the right price. Something like this 30$ one that claims to come with a charge controller and give you 10 to 20 watt hours a day. http://www.amazon.com/Solar-Panel-Charging-Kit-battery/dp/B00EB2VGTW/ref=cm_cd_ql_qh_dp_t combine that with a 12$ battery like this one: http://www.amazon.com/Volt-Alarm-Battery-12v5ah-Hour/dp/B0010Z4MDK/ref=sr_1_2?ie=UTF8&qid=1398201863&sr=8-2&keywords=5ah and you have a lot more power available than you would get from 8 AAs. And you can choose any sized lead acid you want.
But then that's me and I really hate batteries wearing out.