How to store the electric energy produced by solar panels properly?
So, you have panels wired all in series to give up to 8A at 4V.

The first stage will be a boost converter to give you up to 8/3 amps at 12V. You can buy these, which is generally cheaper and much easier than trying to build one. Note that for 8A it'll probably require a fan, and should itself be kept out of the sun to keep cool.

Let's have a look at some references: http://www.powerstream.com/SLA.htm and http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Key facts to take from there are: 


the voltage must be slightly more than 12V to charge the battery, but not too high
total charge duration should be 12-16h


That implies that a 12amp-hour battery should not be charged with more than 1A. That presents a problem for the charger design as you have (in ideal conditions) too much power available from the cells. So you want a boost converter that's current limited to 1A on the output, preferably designed for battery charging.