ordinary mechanical relays arare fast enough to manage 1/1000 second accuracy
have one set of relay cntacts and one set of resistors for each camera. (or two contacts if you needs focus and shutter)
solid-state relays can be non-ideal in interesting ways, bubut the right one could work too,
or you could do it in discrete transistors like this;
simulate this circuit – Schematic created using CircuitLab
(immagine that repeating bit four more times)