Volts don't stack. Four 120v devices connected to the same power supply still draw 120v from that power supply. The differences are the wattage and therefore also the amperage used. You'd need to find out the maximum wattages or amperages for each device, add them together, measure the gauge of the power cable (preferably also the wires between the outlet & breaker box), look up how many amps that cable can handle and see if you've gone over.
If you wire two 15A 120V lines in series, you'll get 15A 240V. If you wire two 15A 120V lines in parallel, you'll get 30A 120V. If you don't understand what either of us are talking about, you should NOT pursue any form of AC electrical, because you'll kill yourself trying to.