Charging 4 x 12V lead acid battery with solar in series or parallel?
If charge time not overly crucial you can charge one battery at a time with one charger and 4 x DPST relays.  

Use of 4 DPDT relays and a single 12V charger works but relays must handle operate current which may be >> charge current, requiring heavier relays.  

Winny's solution of 4 floating chargers is good for mains operation - harder with direct PV panel use.   

You can get DCDC converters with floating output on eg ebay or aliexpress - isolated outputs are MUCH rarer than nonisolated.