I'm assuming you mean a 12V, 4Ah SLA.
What sort of charger, fast or float?
A float charger is simple, just make an LM317 voltage regulator with an output of 13.8V and connect the battery to it. It will take a long time to charge but is simple, cheap and reliable.
A fast charger is slightly more complicated, charge to about 14.8V limiting the current to <4Ah, when the current drops below about 400mA the battery should be either disconnected or allowed to float at 13.8V.