FWIW, my experience has been with a pair of 1/4 inch (6.3 mm) vent holes when staging D motors. I have had good results with that "level" of venting. I don't have analytical notes to back it up (e.g., pressure-volume-time-etc behavior of the interstage gases) but it has worked reliably for gap distances of 1-2-3 inches.
Your setup (three holes, 4 mm) is about 60 percent of the same vent area. If anything, I would recommend that you use at least four, preferably five holes. Or, if you could switch to a 5 mm hole size, three vent holes looks like it would work well. (Based on my experience.) OTOH, your setup might work just fine. If you stick with your plan, please let us know about your flight results?
IMO, the vent holes need to be located (longitudinally) at or slightly above the upper nozzle. I am going to disagree with Gillard at this point, because I think it is indeed important to get rid of the cold air and make a ready path for the hot gas from the booster when using gap staging. I also disagree that particles (of any kind) have much to do with ignition of the upper motor; any burning particles that make it through the next nozzle are nice, but I believe the key ignition mechanism is intense radiant infra-red energy from the hot gas.
Think of an old-fashioned flash bulb going off right next to a sheet of paper and setting it on fire---there are no burning bits thrown onto the paper because the flash bulb is enclosed, but the heat of the flash is still enough to cause ignition. OTOH, maybe I am only giving away my age here (flash bulbs and all that...)
If radiated heat from the booster blow-through is what ignites the exposed black powder in the upper stage nozzle, then obviously you need to get that gas as close to the nozzle as possible. That is why I vote for plenty of venting. I also typically use a forward extension of the booster MMT to reach up within a 1/4 inch or so of the next nozzle, to help contain and focus the blow-through gases toward that nozzle.
I think if you must err, it would be better to go a bit toward the side of "too much" vent area. Definitely would be a bad thing to have "too little" vent area, as that would invite the rapid separation of stages....without ignition.