Switches used in DC applications should be snap action to minimize arcing. Arc suppression diodes should be installed across inductive loads such as relay and contactor coils. Because DC current does not drop to zero like AC does, switches used for DC need to be derated. So the current rating will not increase from 4 to 40. A switch that is loaded above its rating will not immediately self destruct. But its life expectancy will be shortened. For instance, if a switch that is rated for 10,000 cycles at 4 amps is used in an 8 amp circuit, it might only last for 3,000 cycles. The actual lifespan can only be determined by testing. Most switch manufactures have those test results but might not publish them.