Always remember when all else fails look at the instruction manual for your product
But to answer, the blue wire is the sync wire, you wire all the quad products together with the blue wire (it does not need an originating positive/ground connection) and you'll end up with a fantastic synchronization.
Just take note the blue wire is sort of a multi-purpose wire, it's also how you change the follower/leader mode of the light as well as the flash pattern, so it's usually easier to pre-configure all the lights before installation at least as far as leader/follower goes.
Once the lights are in I'll tie them all together then do a 3 second 12v ground to 'reset' everything to the base flash pattern (the 3 second ground is how you change follower/leader but it also defaults to the 1st flash pattern while preserving the color set via the green wire, so all you end up doing is reversing the leader/follower settings of all the lights) and then ground-tap to set the pattern on everything collectively.