On most bikes, there is a single wire coming from the flasher relay to the turn signal switch, and that switch either connects that wire to nothing (not signalling), or to the left turn signal circuit, or the right turn signal circuit, depending on switch position.
If that describes your bike's turn signal circuit, then if you obtain a double-pole normally-open latching pushbutton switch, connect that single wire from the flasher relay to one side of both switch contacts, the left turn signal circuit to the other side of one switch contact, and the right turn signal circuit to the other side of the other switch contact, that will do the job. When the switch is off, it will have no effect, and when the switch is on, it will connect the flasher signal to both the left and right turn signal circuits. Presto, 4 way flashers.
There are some potential gotchas.
If your bike has a single "turn" indicator (rather than a separate one for left and right), it won't work when your 4-ways are on because even in the "on" part of the flash cycle, it won't have voltage across it. Who cares.
If the current draw is beyond what your flasher relay is calibrated for, either the flashing rate will be abnormal or it won't flash (either stay on, or stay off) or it will pop a fuse. You might need to replace the flasher relay with one that does not have the bulb-failure warning feature that stock flasher relays always have. (Same thing people can do when using LED turn signals if not using the "cheat" method i.e. load resistors)
If your bike has a fancy flasher relay with self-cancelling or if the lighting is controlled through a central lighting module (i.e. your switch tells a computer what you want and then the computer turns the lights on and off) then all bets are off, you are on your own.
