Charging four 12 V lead acid batteries in series or parallel?
You can charge them in parallel, assuming your charger has sufficient current capacity.  The charger will be effectively charging a 48 Ah battery.  That should be in range for a normal off the shelf car battery charger.

Even if the charger's current capability is lower than what the batteries could be charged at, as long as it can sustain the float charge current, the batteries will eventually get charged.  Since the float charge current is rather low, pretty much any "car battery charger" should be able to eventually charge these batteries.